Thin-sheet zeolite membrane and methods for making the same

ABSTRACT

Zeolite membrane sheets for separation of mixtures containing water are provided, as well as methods for making the same. Thin, but robust, zeolite membrane sheets having an inter-grown zeolite crystal film directly on a thin, less than 200 micron thick, porous support sheet free of any surface pores with a size above 10 microns. The zeolite membrane film thickness is less than about 10 microns above the support surface and less than about 5 microns below the support surface. Methods of preparing the membrane are disclosed which include coating of the support sheet surface with a seed coating solution containing the parent zeolite crystals with mean particle sizes from about 0.5 to 2.0 microns at loading of 0.05-0.5 mg/cm2 and subsequent growth of the seeded sheet in a growth reactor loaded with a growth solution over a temperature range of about 45° C. to about 120° C.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from and is a divisional of currentlypending U.S. patent application Ser. No. 14/733,735, filed Jun. 8, 2015,which claims priority from and is a continuation-in-part of U.S. patentapplication Ser. No. 13/032,752, filed Feb. 23, 2011 (now U.S. Pat. No.9,079,136, issued Jul. 14, 2015), which claims priority from and is acontinuation-in-part of U.S. patent application Ser. No. 12/817,694,filed Jun. 17, 2010 (now abandoned), and is a continuation-in-part ofU.S. patent application Ser. No. 12/470,294, filed May 21, 2009 (nowU.S. Pat. No. 8,673,067, issued Mar. 18, 2014). U.S. patent applicationSer. No. 12/817,694 also claims priority from U.S. Provisional PatentApplication 61/218,521, filed Jun. 19, 2009. Each of the aforementionedapplications is incorporated herein by reference in their entirety.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract Nos.DE-AC0576RL01830, DE-AR0000138, DE-AR0000372, and DE-FC36-04G098014awarded by the U.S. Department of Energy. The government has certainrights in the invention.

FIELD

The present disclosure relates to membranes, specifically to thin-sheetzeolite membranes, materials and methods of making and using the samefor selective gas separation, such as separation of H₂O molecules.

BACKGROUND

While membrane separation is viewed as an energy and capital efficientprocess, present membrane technologies can often fall short ofperformance requirements for many separation and reaction tasks. Forexample, polymeric membranes can be degraded by hydrocarbons that existin many industrial processes. The polymeric membranes operate atrelative low temperatures. The permeation flux and selectivity of thepolymeric membrane is relatively low. Inorganic and ceramic membranes,including zeolites, supported on porous substrates can provide highpermeation flux and separation selectivity while exhibiting chemical andthermal stability. However, they are typically fragile and are generallymade as membrane tubes with a surface area packing density much smallerthan the polymeric membranes.

Zeolites are made of in-expensive raw materials and possess some uniqueproperties, such as molecular sieving functions and stability.Particularly, the zeolite framework is stable in hydrocarbon or organicsolvent medium and at elevated temperatures. These are very desirableattributes as a membrane material. Thus, there has been a stronginterest to making zeolite membranes worldwide. The feasibility ofmaking zeolite membranes and their unique molecular-sieving functionswere demonstrated in late 1990s and early 2000s. The reported flux andselectivity of a zeolite membrane can be one or two orders of magnitudehigher than the polymeric membranes for solvent dehydration. Mostlysmall ceramic disks and single-hole tubes were used as a support toelucidate membrane preparation and structures, and to demonstrate basicseparation process concepts. Such supporting materials served thoseresearch purposes well. For actual membrane product development,however, novel product designs and manufacturing processes are needed toproduce large membrane areas suitable for industrial applications at acost competitive with existing separation technologies.

For recent two decades, there has been research publications aroundNaA-type and other zeolite membranes. The NaA membrane supported onceramic alumina tubes has been commercialized for ethanol and solventdehydration. Instead of single holes, several holes can be made into onemembrane tube to increase the membrane area packing density. However,the tubular membrane cost cited in the literature is viewed too high forwidespread usage. Membrane area packing density is another key issue toapplications requiring large membrane areas.

The area packing density increases with decreasing the tube diameter.Exploratory studies of capillary inorganic membrane tubes have beenreported with an attempt to achieve dramatic enhancement of the membranearea packing density. The feasibility to deposit a quality NaA membraneon an alumina hollow fiber of 1.2 mm O.D×0.6 mm thickness was shown. Theceramic capillary tubes tend to be brittle. An alternative isdevelopment of ceramic monolithic membranes. In the monolithic membranebody, a number of small membrane channels (<1.0 mm) are embedded in asturdy, porous ceramic matrix so that making and packaging ofindividual, fragile capillary tubes is avoided, and manufacturingproductivity of the membrane can be enhanced at the same time. Themonolithic designs represent promising progress toward getting thesurface area packing density of inorganic membranes close to polymerichollow fiber membranes.

The attempt to make flat sheet zeolite membranes has been reported inthe literature using sintered porous metal plates and metal meshes as asupport. However, those porous metal supporting structures had roughpores and were too thick. The thick support is associated with highmetal material costs and mass transport resistance. The rough pore ofthe support requires thick coating of modification and/or membranelayer. The thick coating adds membrane preparation complexity andpresents potential adhesion/crack problems.

Inorganic membranes can have distinct advantages regarding resistance todegradation by various chemicals, stability at elevated temperatures,and high permeation flux and selectivity. Inorganic membranes aretypically tubular in shape or are thick coatings on thick substrates.Tubes are commonly associated with relatively lower surface area packingdensity and higher cost per unit membrane area and engineering cost.Thick membranes have traditionally been important to seal defects suchas pinholes and void structures present in thin membrane films, whichare typically caused by less-than-ideal preparation procedures for thesubstrate structure and membrane. The thicker membranes generallyprovide low permeation flux and are associated with adhesion problemswhen the membrane and substrate are two different kinds of materials.For example, thermal mismatch between the membrane coating and substratematerial can become pronounced with increasing the membrane thickness. Athick substrate (1 mm or above) is typically used in the conventionalzeolite membrane synthesis due to the strength requirement. Porousceramic tubes or disks are fragile and can easily be broken if madethin. Conventional metal screens or foams have large pores and are weakif made thin. Furthermore, thick substrates can increase the cost andweight of the membrane structure. Thicker substrates also imposeadditional resistance for the permeate to move through. These issues,and others, have been a major barrier hindering the development ofefficient, thin, inorganic membranes having widespread applicability.

SUMMARY

To reduce membrane fabrication cost and achieve high separationperformances at the same time, disclosed herein are thin-sheet zeolitemembranes. The disclosed zeolite membrane are as thin as metal foils andpapers so that the membrane sheets can be manufactured with highthroughput at competitive costs and packaged into membrane modules ofarea packing density as high as polymeric membrane sheets. This uniquecombination of performance attributes has not been obtained yet withconventional ceramic or polymeric materials alone. As such, methods tomanufacture thin-sheet zeolite membranes by direct deposition of a pure,continuous, inter-grown zeolite crystal layer on thin porous supportsheets are disclosed. The fundamental feasibility was studied with modelsimulation of the stress distribution. The zeolite film thickness andits penetration depth into underneath support pores are importantmembrane design parameters. For a given membrane/support pair ofdifferent thermal expansion coefficients, it is desirable for these twoparameters to be maintained below a certain value to avoid membranecracks caused by the thermal stresses, which could be induced duringmembrane preparation and usage. In particular, the inventors evaluated a50 μm-thin porous metal sheet formed as a membrane support and NaA (4A)as an exemplary zeolite framework. Molecular separation functions of theresulting membranes are characterized with H₂O/air gas-phase separationtests. Selective removal of water vapor from humid air has largeapplication opportunities for air dehumidification in buildings and inindustrial processes as well. In addition, selective H₂O/air separationpresents more stringent selectivity requirement than separation of H₂Ofrom other lager molecules such as alcohols. Secondary growth wasdemonstrated as one effective method to make the proposed membrane. Thesupport surface textures are also vital for formation of a continuous,dense zeolite membrane layer. The support surface needs to besubstantially free of any holes, pores, and cavities above 10 μm. Thepreferred support porosity is from 20% to 50%. H₂O permeance of 1.0E-5mole/s/m²/Pa with H₂O/N₂ separation factor above 3000 can be obtainedwith an optimum membrane.

The present invention teaches thin but robust zeolite membrane sheetscomprising a zeolite membrane formed directly on a thin metal supportsheet. The membrane sheets exhibit combined performance attributes,which include high flux and selectivity, chemical and thermal stability,mechanical flexibility and strength, and high surface area packingdensity, that are not be provided by conventional polymeric, ceramic ormetallic membrane products.

Embodiments of the present invention include thin zeolite membranesheets and methods of making the same. The zeolite membrane sheetscomprise a zeolite membrane layer having a thickness of less than orequal to 20 μm, such as less than 10 μm (e.g., between 2 μm and 10 μm, 3μm and 9 μm, 3 μm and 5 μm, 4 μm and 8 μm, including 1 μm, 2 μm, 3 μm, 4μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm) formed above a thin porousmetal support sheet having a thickness less than or equal toapproximately 200 μm, such between 20 μm and 200 μm and the zeolitemembrane layer having a penetration depth of less than about 20 μm, suchas 5 μm (e.g., such as between 2 μm and 10 μm, 3 μm and 9 μm, 3 μm and 5μm, 4 μm and 8 μm, including 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8μm, 9 μm, or 10 μm) formed below the support surface. In someembodiments, the zeolite membrane layer has a thickness of less than 20μm and is formed on a thin porous metal support sheet having a thicknessless than or equal to approximately 200 μm. The zeolite membrane layeris formed by applying a uniform seeding layer to a bare metal surface ofthe porous metal support sheet at a loading of 0.03 to 0.5 mg of seedingcrystals per cm² of the support surface. In one example, a support sheetis substantially free of any holes, pores and/or cavities greater than10 μm and the support sheet has a porosity from 20% to 50%.Substantially-free is a phrase used to describe a condition in which nocavities and/or cracks can be visually seen on the support sheet by aperson with normal vision, no light transmission through the supportsheet can be seen when the support sheet is against a visible lightsource, and no large holes or pores greater than 10 μm can be seen whena sample of the support surface is observed under microscopy (scanningelectron microscopy (SEM) or high-resolution optical microscopy) or theprobability for observation of such large defects under microscopy isless than 1%. For example, metal meshes that can readily transmitvisible light are not suitable support for present invention. Thicksintered metal plates and porous metal foams that do not transmitvisible light, but show rough pores greater than 10 μm under microscopyeven though their average pore sizes can be well below 10 μm are not thesuitable support for present invention. In some examples, the seedinglayer comprises parent zeolite crystals having an average particle sizeless than or equal to 3 μm, such as between 0.1 to 2.0 μm, as seeds forzeolite membrane growth. The particle size of seeding crystals istypically determined by use of a standard particle size analyzer(Micotrac) when they are dispersed in a seed coating solution. Thelarger particles in the seed coating solution typically sediments on thebottom of the bottle and are not used for seed coating. The seed coatingsolution is preferably wettable on the support surface so that certainpenetration of the seed coating solution into the support pores canoccur to let some seeding crystals sit inside the support pores. Theseed coating solution is typically water-based, and some solvent andadditives may be added to modify the surface tension and contact angleof the solution on the support surface. The seed coating solution can beprepared by dispersing the parent zeolite crystals, which is often inpowder form, into a fluid at solid loading of 1 to 6 wt. %. The coatingsolution needs to be homogeneous and stable during the seed coatingprocess. If the parent zeolite powder comprises distinctive, smallparticles, the zeolite powder may be simply dispersed by stirring. Ifthe parent zeolite powder comprises agglomerates, the powder can beball-milled or attrition-milled in the dispersing fluid into the meanparticle size of about 0.2 to 2.5 μm. Crystal purity of the parentzeolite material to be used as seeding crystals needs to be confirmed byXRD analysis. The ball-milling or attrition-milling is controlled insuch a degree that no significant degradation of the parent zeolitecrystal occurs. The seed coating is preferably conducted by spray ordip-coating so that presence of excessive coating solution on thesupport surface is minimized. In some examples, the seeding layercomprises zeolite crystals having an average diameter less than or equalto 2.0 μm. For example, an exemplary porous metal support sheetcomprises an average pore size of less than 3 μm, a porosity between 30%and 45% and a thickness of less than or equal to 200 μm, such as between25 μm and 200 μm and is coated with a seeding layer comprising zeolitecrystals between 1.0 μm to 1.4 μm. In some examples, methods ofpreparing the membrane are disclosed which include coating of thesupport sheet surface with a seed coating solution containing the parentzeolite crystals with mean particle sizes from about 0.1 to 2.0 micronsat loading of 0.05-0.5 mg/cm² without forming a continuous, distinctiveseed coating layer, which can be determined by change of the supportsurface appearance from metallic lusters into white-colored paintings.The optimum seed loading on the support surface is that the supportsurface is fully decorated by the seeding crystals and support pores arefilled by the seeding crystals, but there is no formation of thickcoating layers, patches or spots. The thick coating can readily causedefects in the membrane growth.

The seeded support sheets are loaded into a hydrothermal growth reactorto form a dense, continuous, inter-grown zeolite membrane layer. Themembrane sheets in the growth reactor are fully immersed in the growthsolution. The growth solution is prepared in proper procedures withproper compositions specifically for growth of the targeted zeoliteframework. The preparation procedures and compositions for growthNaA-type, Faujasite-type, and modern framework inverted (MFI)-typezeolite membranes will be described in the respective examples. Thegrowth solution needs to be homogeneous and stable. The stability of thegrowth solution can be assessed by no significant phase segregation orprecipitation in one hour when the solution is left still. A number ofseeded sheets can be loaded into one reactor by having a gap between thesheets, such as 1-mm gap. The seeded sheets are preferably verticallyoriented in the reactor. The reactor is built and operated in such a waythat the temperature distribution in the growth zone is uniform. Forexample, a planar reactor can be made by applying uniform heating on thetwo reactor plates with a small spacing, and the growth solution isheated up by thermal conduction. The growth conditions are controlled toavoid under growth or over growth. In the under growth, a continuousmembrane layer is not formed and the zeolite membrane crystal is notfully grown. In the over growth, the zeolite membrane layer is grown toothick and/or there are too much deposition of crystals and/or particlesfrom bulk solution onto the membrane layer. The reaction heating rate istypically 1° C./min. For growth of NaA-type and Faujasite-type zeolitemembranes, the preferred growth temperature and holding time at thegrowth temperature are 60° C. to 110° C. and 0 to 15 hours,respectively. For growth of MFI-type zeolite membrane, the preferredgrowth temperature and holding time are 120° C. to 160° C. and 1 to 4hours, respectively.

In other examples, an exemplary porous metal support sheet with at leastone surface comprises an average pore size of less than 3 μm withsubstantially free of pores and defects greater than 10 μm, a porositybetween 15% and 55%, and a thickness of less than or equal to 200 μm.The pore size of a support surface is preferably determined by themercury porosimetry or microscope. As a membrane support of presentinvention, knowing the average pore size is not sufficient, and thesupport surface must be substantially free of defects greater than 10μm, preferably the probability of presence of such large defects on asupport surface is <1%. The support sheet coated with the seedingcrystal is immersed in a zeolite growth solution to hydrothermally forman inter-crystal growth layer and complete the zeolite membrane layer.The inter-crystal growth layer incorporates the seeding layer, comprisesthe same zeolite as the zeolite crystals, and completes formation of thecontinuous zeolite membrane layer to a thickness less than or equal toapproximately 20 μm, such as less than or equal to 10 μm, includingbetween 1.0 μm and 5 μm. The membrane growth can be characterized bygrowth weight gain, and the preferred growth weight gain is 0.3 to 3.0mg/cm2 of the support surface. The membrane thickness is checked bycross-section analysis of the membrane sheet under microscopy. Since theporous metal substrate and zeolite membrane layer are two differentmaterials and have different chemical compositions and coefficients ofthermal expansion, the thinness of the zeolite membrane layer iscritical to avoid formation of cracks by withstanding stresses inducedduring membrane processing and membrane separation operation. There isno chemical bonding between the zeolite crystals and support. Having anadequate growth penetration depth of zeolite membrane layer intounderneath support is necessary to have good membrane adhesion bymechanical interlocking. This is controlled by seed coating and growth.There is a fraction of seeding crystals in the seed coating solutionwith smaller sizes than the support surface pores to sit in the supportpore. The seed coating is not too thick and the growth solution is nottoo concentrated so that secondary growth can start from exterior poresof the support, not just on the outer surface of the seed coating layer.

In some embodiments, the zeolite material comprises a water selectivezeolite, a hydrocarbon-selective zeolite, or an alcohol-selectivezeolite. Exemplary water-selective zeolites include, but are not limitedto NaA-type framework that may be exchanged with different metal ions,and Faujsite-type frameworks (NaX and NaY). Exemplaryhydrocarbon-selective and alcohol-selective zeolites include, but arenot limited to pure silicalite, titanium silicate, and MFI-type andY-type zeolite frameworks.

The metal support sheet preferably comprises porous Ni, a porous Nialloy, or a porous steel. However, porous metal support sheets havingone support surface that has an average pore size less than 3 μm withlarge pores and defects greater than 10 μm less than 1% of the supportarea, a porosity between 15% and 55%, such as between 30% and 45%, and athickness less than or equal to 200 μm, such as 20 μm to 200 μm, can besuitable when the zeolite is formed directly on the bare surface of thesupport sheet without an intervening transition layer such as a ceramicmaterial having a different composition than the zeolite membrane.

In some embodiments, the seeding layer can be formed by providingapplication of single solution containing the parent zeolite seedingcrystals, such as by spraying. Multiple times of spraying may beperformance to enhance uniformity of the deposition with intermittentdrying. Spray coating is developed as a simple method to lay down theseeding crystals on the support sheet. An appropriate seed loadingsurface density (mg/cm²) is necessary to grow a quality membrane withoutgood adhesion and high permeance without defects and cracks. Forexample, the membrane layer thickness between 1 and 20 μm above thesupport surface and penetration depth greater than zero and less than<6.0 μm below the support surface were found with the membranes that arestable exhibiting both high permeance and selectivity when the membranesheet is subject temperature changes during growth, post-treatment andapplication. In some examples, the disclosed methods include coating thesupport sheet surface with a seed coating solution containing the parentzeolite crystals with mean particle sizes from about 0.1 to 3.0 micronsat loading of 0.05-0.5 mg/cm² and subsequent growth of the seeded sheetin a growth reactor loaded with a growth solution over a temperaturerange of about 60° C. to about 160° C. for 0 to 7 hours.

In other embodiments, the seeding layer can be formed by performingmultiple applications of the zeolite crystals in a graded structure.Accordingly, initial zeolite seeding particles, closest to the porousmetal support sheet, would be relatively larger, having averagediameters between 1.0 and 3 μm, and the seeding particles in subsequentapplications are smaller less than 1.0 μm. Some penetration of the seedcrystals into the underneath support pores is preferred in order toobtain mechanic inter-locking between the zeolite membrane and thesubstrate sheet. However, significant filling of the support pores bythe seeding particles should be avoided to minimize the stress inducedto the support and minimize mass transport resistance across thesupport. The thin-sheet zeolite membranes of present invention can bepackaged into a high throughput membrane module to conduct actualseparation processes. Since the thin-sheet zeolite membrane can be madeas thin and flexible as polymeric membrane sheets, the zeolite membranesheet can be made like polymeric membrane modules, such as spiral-woundmodule and plate-type module. Since the zeolite/metal sheet has goodmechanical strength and rigidity, new membrane modules may be designedand built according to specific application needs. Since the zeolitemembrane sheet is so thin, a large membrane area can be packaged perunit module volume. For example, if 50 μm-thick zeolite membrane sheetsare arranged with 1.0 mm spacing in a module, a membrane area packingdensity can be as high as 1000 m²/m³.

Molecular separation with the thin-sheet zeolite membrane is carried outby creating a differential in partial pressure or chemical potential ofthe permeate molecule between the two sides of the membrane. Themembrane side of the sheet is exposed to a feed stream, while thebackside of the sheet acts for the permeate. In one common practice, thepermeate side can be maintained a lower pressure than the feed side. Insuch an arrangement, the membrane sheet is strong enough to withstandthe pressure gradient. In another common practice, the permeate side ofthe membrane can be swept by a fluid when the two sides of the membranesheet are kept at the similar pressure.

The presently disclosed membranes can be used in association withmembrane separation and membrane reactors resulting in higher energyefficiency and/or lower capital costs. There are a range of energyconversion and environmental applications, such as, atmospheric airdehumidification for buildings and people comfort in other livingsystems, process water removal, ethanol or high alcohol production frombiomass, CO₂ capture from a gas mixture, and removal of particularhydrocarbons from solvents or hydrocarbon mixtures.

The purpose of the foregoing abstract is to enable the United StatesPatent and Trademark Office and the public generally, especially thescientists, engineers, and practitioners in the art who are not familiarwith patent or legal terms or phraseology, to determine quickly from acursory inspection the nature and essence of the technical disclosure ofthe application. The abstract is neither intended to define theinvention of the application, which is measured by the claims, nor is itintended to be limiting as to the scope of the invention in any way.

Various advantages and novel features of the present invention aredescribed herein and will become further readily apparent to thoseskilled in this art from the following detailed description. In thepreceding and following descriptions, the various embodiments, includingthe preferred embodiments, have been shown and described. Includedherein is a description of the best mode contemplated for carrying outthe invention. As will be realized, the invention is capable ofmodification in various respects without departing from the invention.Accordingly, the drawings and description of the preferred embodimentsset forth hereafter are to be regarded as illustrative in nature, andnot as restrictive.

The foregoing features of the disclosure will become more apparent fromthe following detailed description, which proceeds with reference to theaccompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B each provide a schematic of an exemplary membrane.

FIGS. 2A-2D provide phase-field generated porous structures withdifferent pore sizes, but the same pore volume fraction (50%)(dimensionless pore size ϕ_(p) (d_(p)/h0)=0.12 for case 1, 0.25 for case2, 0.81 for case 3, and 1.1 for case 4).

FIG. 3 provides an exemplary constructed film structure for thermalstress analysis.

FIGS. 4A-4E provide schematics of methods of use of an exemplarymembrane for water removal.

FIGS. 5A and 5B illustrate probability distribution of stress in theregion near the interface for the four different porous Ni substratecases, h₀=10, h₁=4, in unit of μm and ΔT=200K FIG. 6 provides twodimensional stress distributions on the center plane A for the caseΔT=200K, and h₁=4, and different h₀=4, 8, and 12. in unit of μm.

FIG. 7 provides two dimensional stress distributions on the center planeA for the case ΔT=200K, and h₀=10, and different h₁=2, 4, and 6. in unitof μm.

FIGS. 8A and 8B provide probability distribution of stress in the regionnear the interface for the cases, ΔT=200K porous structure (Case 3),h₀=10, and different h₁=0, 2, 4, 6, and 8. in unit of μm.

FIGS. 9A-9E illustrate temperature dependence of maximum stresses andfailure modes in the film.

FIGS. 10A and 10B illustrate pore size distribution of porous nickelsheets of two different thicknesses FIGS. 11A-11D are scanning electronmicroscope (SEM) images illustrating pore structures of 50 μm-thickporous Ni sheets prepared and used for zeolite membrane preparation.

FIGS. 12A and 12B illustrate particle size distribution of ball-milledparent zeolite powder for seed coating.

FIGS. 13A-13D are SEM images illustrating textures of a porous Ni sheetseeded with micro-A and nano-A coating solutions sequentially (seededsupport for growth of membrane 27-26-4, 3 wt. % solid loading in bothseeding solutions, 0.278 mg/cm² total seed loading).

FIGS. 14A and 14B are digital images of surface textures of membrane anexemplary membrane (#9-7-4 membrane).

FIGS. 15A1-15E2 provide cross-sectional views of membranes grown underthe same conditions on supports of different seeding crystal loadings(3.5 hours hold at 90° C.).

FIG. 16 is a graph illustrating particle size distribution of macro-Aparent powder milled at different conditions for seed coating.

FIGS. 17A-17D are SEM images illustrating surface textures of porous Nisheets seeded with different parent NaA powder.

FIGS. 18A-18D are SEM images illustrating surface and interfacestructures of zeolite membranes grown on the support seeded with onetype of seeding crystals.

FIG. 19 is a tracing illustrating XRD patterns of NaA membranes grown onthe support seeded with one type of seeding crystal.

FIG. 20 is a comparison of tracing obtained from NaA membranes grownwith different holding time at 90° C.

FIG. 21A-21F are SEM images for surface and polished cross-section ofmembranes grown at different holding time at 90° C. on the supportseeded with identical seeding solution and procedures.

FIG. 22 is a set of digital images illustrating the mechanical strengthand flexibility of a 50 μm-thick 12 cm×12 cm NaA/metal sheet membrane.

FIGS. 23A and 23B are SEM images of surface textures of 134 μm-thickporous Ti support as-received and after seed coating.

FIGS. 24A-24D are SEM images of surface textures of 150 μm-thick porousTi support as-received, after seed coating, and after secondary growth.

FIGS. 25A and 25B are SEM images of surface textures of 43 μm/250 μmporous Ti support after seed coating and growth.

FIGS. 26A-26C are SEM images of surface textures of 43 μm/140 μm porousTi support as-received, after seed coating, and after secondary growth.

FIG. 27A-27C are SEM micrographs of an embodiment of a zeolite membranesheet in which the membrane layer was prepared with a seeding layer andusing a template-free synthesis solution.

FIGS. 28A-28D are SEM micrographs of an embodiment of ahydrocarbon-selective and/or alcohol-selective zeolite membrane sheet inwhich the membrane layer was prepared with a seeding layer and using atemplate-containing synthesis solution.

FIG. 28E is a X-ray diffraction spectrum obtained from an embodiment ofa hydrocarbon-selective and/or alcohol-selective zeolite membrane sheetin which the membrane layer was prepared with a seeding layer and usinga template-containing synthesis solution.

FIGS. 29A-29D are SEM micrographs of an embodiment of a water-selectivezeolite membrane sheet in which the zeolite membrane is formed directlyon the support sheet without a seeding layer.

FIG. 29E is an X-ray diffraction (XRD) spectrum obtained from anembodiment of a water-selective zeolite membrane sheet in which thezeolite membrane is formed directly on the support sheet without aseeding layer.

FIG. 30 is a plot of water permeation flux and water/ethanol selectivityfactor as a function of temperature using an embodiment of awater-selective membrane sheet.

FIG. 31 is a plot of water flux and water/ethanol selectivity as afunction of weight percent water in the feed mixture using an embodimentof a water-selective membrane sheet.

FIG. 32 is a plot of water flux and water/ethanol selectivity as afunction of time on stream using an embodiment of a water-selectivemembrane sheet.

FIGS. 33A and 33B are illustrations depicting an embodiment of amini-channel membrane module according to the present invention.

FIG. 34 is an illustration depicting an embodiment of a plate-typemembrane module according to the present invention.

FIG. 35 is a schematic illustrating an exemplary method for preparingFaujasite membranes according to the present invention.

FIGS. 36A and 36B are SEM images of synthetic NaY and NaX received froma commercial source (Wako) as seeding crystals.

FIG. 37 is an XRD analysis of as-received NaY powder used for NaYseeding crystal preparation.

FIG. 38 is an XRD analysis of as-received NaX powder used for NaXseeding crystal preparation.

FIGS. 39A-39C are SEM images illustrating crystal morphologies ofFaujasite powder grown with different holding time at 90° C. (sample#60037-38).

FIG. 40 provides XRD patterns of Faujasite powder grown under conditionsdisclosed herein compared to commercial NaX.

FIGS. 41A-41H are SEM images illustrating textures of four seed-coatedsurfaces and membranes grown with 3.5-hour hold time.

FIGS. 42A-42D are SEM images of membranes grown on 8-hour dryseed-coated surfaces with different holding time.

FIG. 43 is an XRD analysis of membranes grown over the support seededwith wet zeolite particles for 3.5 hours.

FIG. 44 is an XRD analysis of membranes grown on the support seeded with8-hour grown zeolite powder for different times.

FIGS. 45A-45L are SEM images illustrating surface textures ofion-exchanged NaA/Ni membranes.

FIGS. 46A-46G are SEM images illustrating the textures of the NaAmembranes modified by different surface reaction (coupons sampled frommembrane sheet #60037-11-5).

FIGS. 47A-47C are graphs illustrating elective H₂O permeance propertiesof NaA/Ni membrane modified by Ag ion exchange (membrane ID61794-13-Ag).

FIGS. 48A and 48B are graphs illustrating gas permeance of parent NaA/Nimembrane (membrane ID 60037-89-2).

FIGS. 49A-49C are SEM images illustrating surface textures ofas-prepared, tested, and exchanged and tested NaA/Ni membranes (membraneID 60037-89-2).

FIGS. 50A and 50B are graphs illustrating gas permeance of Agion-exchanged NaA/Ni membrane (ID61794-41-4).

FIGS. 51A and 51B are graphs illustrating gas permeance of Nafion N115-3membrane.

FIG. 52 is a schematic illustrating exemplary process steps of MFI-typemembrane preparation of present invention FIGS. 53A and 53B provideprocess flow diagrams for application of ethanol-selective MFI-typemembrane to concentration of dilute alcohols in water.

FIGS. 54A and 54B are SEMs illustrating morphologies of seeding crystalsprepared for MFI-type membrane growth.

FIGS. 55A-55D are SEM/EDS analysis of seeded support sheets.

FIGS. 56A-56F are SEMs of micro-structures of MFI-membrane grown at 120°C.

FIG. 57 illustrate XRD patterns of MFI-membrane grown with calcinedseeds.

FIG. 58 illustrate XRD patterns of MFI-membrane grown withas-synthesized seeds.

FIG. 59 is a graph illustrating ethanol/water separation with asilicalite membrane heated at 400° C.

FIG. 60 is a graph illustrating ethanol/water separation bypervaporation over a silicalite membrane.

DETAILED DESCRIPTION I. Abbreviations and Terms i. Abbreviations

ρ_(s)=packing density of seed coating, g/cm³

ρ_(m)=packing density of membrane coating, g/cm³

δ_(s)=nominal seed coating thickness, μm

δ_(m)=nominal membrane coating thickness, μm

ω_(s)=seed loading density, mg/cm²

ω_(m)=membrane growth density, mg/cm²

Δp_(i)=partial pressure differential of species i between the feed andpermeate side

F_(i)=permeation flow rate of specie i, mol/s

J_(m)=permeation flux, Kg/m²/hr

P_(i)=permeance of specie i, mol/m²/s/Pa

S_(ij)=separation factor of specie i to j

SA_(m)=working surface area of membrane

SA_(s)=area of the seed-coated support sheet.

SA_(m)=area of the membrane grown, cm²

t=testing duration time to collect W_(p)

W_(p)=amount of liquid condensed in the liquid N₂ trap from the permeate

W_(m)=weight of the sheet after growth, mg

W_(o)=weight of bare support sheet, mg

W_(s)=weight of seed-coated sheet, mg

W_(o)=weight of bare support sheet, mg

x_(i)=molar fraction of specie i in feed side

x_(j)=molar fraction of specie j in feed side

y_(i)=molar fraction of specie i in permeate side

y_(j)=molar fraction of specie j in permeate side

ii. Terms

Unless otherwise noted, technical terms are used according toconventional usage. Further, unless otherwise explained, all technicaland scientific terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this disclosurebelongs. The singular terms “a,” “an,” and “the” include pluralreferents unless context clearly indicates otherwise. Similarly, theword “or” is intended to include “and” unless the context clearlyindicates otherwise. It is further to be understood that all base sizesor amino acid sizes, and all molecular weight or molecular mass values,given for nucleic acids or polypeptides are approximate, and areprovided for description. Although methods and materials similar orequivalent to those described herein can be used in the practice ortesting of this disclosure, suitable methods and materials are describedbelow. The term “comprises” means “includes.” The abbreviation, “e.g.”is derived from the Latin exempli gratia, and is used herein to indicatea non-limiting example. Thus, the abbreviation “e.g.” is synonymous withthe term “for example.”

Unless otherwise indicated, all numbers expressing quantities ofingredients, properties such as molecular weight, percentages, and soforth, as used in the specification or claims are to be understood asbeing modified by the term “about.” Accordingly, unless otherwiseindicated, implicitly or explicitly, the numerical parameters set forthare approximations that may depend on the desired properties soughtand/or limits of detection under standard test conditions/methods. Whendirectly and explicitly distinguishing embodiments from discussed priorart, the embodiment numbers are not approximates unless the word “about”is recited.

All publications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entireties. Incase of conflict, the present specification, including explanations ofterms, will control. In addition, all the materials, methods, andexamples are illustrative and not intended to be limiting. In order tofacilitate review of the various embodiments of the disclosure, thefollowing explanations of specific terms are provided:

Absorption: A physical or chemical phenomenon or a process in whichmolecules in a gas phase are taken up into the bulk of a fluid of othermatter, such as liquid fluid or solid-state fluid. Absorption is adifferent process from adsorption, since the molecules are taken up inthe bulk of other matter, not by the surface of other matter. A moregeneral term is sorption, which covers adsorption, absorption, and ionexchange.

Liquid Fluid: A fluid that has the particles loose and can freely form adistinct surface at the boundaries of its bulk material. Examples ofliquids include water and oil.

Membrane support: The membrane support is a structure having a surfacefor deposition or coating of a selective membrane. The membrane supportis permeable so that a molecule separated by the membrane can readilypermeate through the membrane support. The membrane support providesmechanical integrity for a supported membrane.

Moisture: Any type of precipitation. In one example, moisture refers tothe presence of water in air and gases, often in vapor phase.

Permeance or permeation: The degree to which a material admits a flow ofmatter or transmits another substance with unit of driving force.Permeable materials are those through which gases or liquids may pass.Membranes are one type of permeable material and are composed of thinsheets of natural or synthetic material. Frequently, membranes exhibitdifferent permeances—e.g., permeation rates—for different chemicalspecies. In this regard, permselectivity is the preferred permeation ofone chemical species through a membrane with respect to another chemicalspecies. Permselectivity of the desired permeate with respect to anotherchemical species is calculated as the ratio of the permeance of thedesired permeate to the permeance of the other chemical species.Permselective membranes are promising in a variety of applicationsincluding gas separation, electrodialysis, metal recovery, pervaporationand battery separators.

Pore: One of many openings or void spaces in a solid substance of anykind that contribute to the substance's porosity. Porosity is a measureof the void spaces or openings in a material, and is measured as afraction, between 0-1, or as a percentage between 0-100%.

Porous: A term used to describe a matrix or material that is permeableto fluids. For example, a porous matrix is a matrix that is permeated byan interconnected network of pores (voids) that may be filled with afluid (such as a liquid or gas). In some examples, both the matrix andthe pore network (also known as the pore space) are continuous, so as toform two interpenetrating continua. Many materials such as cements,foams, metals and ceramics can be considered porous media. In oneexample, a porous matrix is a nickel matrix prepared by the method offabrication disclosed herein.

Selective Permeation: A process that allows only certain types ofmolecules or ions to pass through a material, such as a membrane. Insome examples, the rate of passage depends on the pressure,concentration, and temperature of the molecules or solutes on eitherside of the membrane, as well as the permeability of the membrane toeach solute. Depending on the membrane and the solute, permeability maydepend on solute size, solubility, or other chemical properties. In oneexample, the membrane is selectively permeable to H2O as compared to O2.

Solubility: A physical property of a liquid multi component systemdescribing its ability to dissolve a substance, the solute, at aspecific temperature and pressure from another phase. Solubility ismeasured as the maximum quantity of a substance that may be dissolved inanother, for example it is the maximum amount of solute that may bedissolved in a solvent.

II. Overview of Several Embodiments

The following description includes the preferred best mode as well asother embodiments of the present invention. It will be clear from thedescription of the invention that the invention is not limited to theillustrated embodiments but that the invention also includes a varietyof modifications and embodiments thereto. Therefore the presentdescription should be seen as illustrative and not limiting. While theinvention is susceptible of various modifications and alternativeconstructions, it should be understood, that there is no intention tolimit the invention to the specific form disclosed. On the contrary, theinvention is to cover all modifications, alternative constructions, andequivalents falling within the spirit and scope of the invention asdefined in the claims.

Rather than using nano-metallic particles to form the thin, porous metalsupport sheets, embodiments of the present invention start withless-ductile metal precursor materials such as oxides, hydroxides,hydrides, and metal-organics that can be acquired from commercialsources at bulk quantities. The less-ductile metal precursors in powderform after milled to suitable particles sizes such as micrometers andsubmicrometers are mixed with pore formers and other additives to form ahomogenous slurry batch at micro- and nano-scale mixing. The slurry iscast into plates or sheets. The green sheet is dried at low temperaturesto generate some-pores inside the body, which allows gas permeate insidein the subsequent step. Then, the metal precursor(s) is converted intometallic state or alloy by an appropriate high-temperature reactionprocesses, such as, direct reduction by H₂ gas at temperatures above750° C. The metal crystal size and sintering can be controlled by thereaction conditions (temperature, time, and gas purity). A continuoushydrogen gas flow in the reduction reactor is preferred. This processstep results in a metallic backbone of uniform pore sizes at micrometeror sub-micro-meter level. Optionally, the porous metal sheet can beannealed by applying a load onto the sheet during annealing to keep thesheet flat and minimize warping. The support sheet synthesis techniquedescribed above allows decoupling of material forming process at atomic,nano-, micro-, and macro-scales. Different from traditional metalfoaming and sintering processes, embodiments of the present inventionuse non-metallic precursor materials, produce smaller and uniform pores,and utilize the porous network for tailoring of the metal grain andbonding. The porous metallic sheets can be made symmetric by use ofsingle green tape, i.e., the same or similar pore structures on bothside of the metal sheet. The porous metallic sheets can be made with agraded structure by use of laminated green tapes with differentcompositions, i.e., different pore structures on the two surface of themembrane sheet. Once the support sheet is formed, it is available forformation of a zeolite membrane layer on at least one side of the sheet(such as the front side). In the examples to demonstrate this invention,50 μm-thick Ni alloy sheets of symmetric pore structures were made byuse of single green tape, which was found to be a simple process.As-prepared metal sheets were examined prior to their usage for zeolitemembrane deposition. First, the support sheets with any visible cracks,holes, marks, large cavities, and inclusion of large particles wererejected. Second, loose particulate on the membrane surface was removedby vacuum cleaning. Third, the metal sheet allowing visible light to gothrough was rejected. Fourth, the metal sheet that breaks at 90-degreebending was rejected. In one example, a zeolite membrane layer is growndirectly on the porous metal support sheet having seeding crystals. Theseeding crystals can be deposited or coated on the support by use of aseed coating solution comprising parent zeolite crystals of suitablesizes being dispersed in a water-based medium. The solution medium orcarrier of the seeding crystal should be wettable on the support surfaceto be coated. The crystal phase of the seeding crystal is the same asthe crystal phase of the zeolite membrane to be grown. While variouszeolite materials can be suitable depending on the processing conditionsand desired separations, preferably, the zeolite material for the seedcrystals and the membrane layer are 4A (or NaA-type), Faujasite-type,MFI-type, or silicalite. An exemplary zeolite loading in the coatingsolution is 1-6 wt %, although other loadings can be suitable andappropriate. The proper particle size and solid loading of the seedingcrystals are used to obtain a homogenous seed coating solution. Theseeding solution can be made by sonication, ball-milling or attritionmilling to break up larger particle agglomerates of the parent zeolitepowder, resulting in a stable and uniform suspension. The large particlein the seed coating solution may be filtered out prior to coating.

In some instances, deposition of seeding crystals on the support isconducted by use of a single seed coating solution with mean particlesfrom 0.1 to 3.0 μm, which is prepared out of one single source of parentzeolite powder. For example, methods of preparing the membrane aredisclosed which include coating of the support sheet surface with a seedcoating solution containing the parent zeolite crystals with meanparticle sizes from about 0.5 to 2.0 microns at loading of 0.05-0.5mg/cm2 and subsequent growth of the seeded sheet in a growth reactorloaded with a growth solution over a temperature range of about 50° C.to about 160° C. for 0 to 6 hours.

In other instances, multiple seeding solutions can be prepared eachcomprising a different average particle size or seeding crystal sizes.The seed coating can be conducted with multiple seeding solutions toavoid excessive penetration of the seeding crystals inside the supportpores and excessive accumulation of the seeding crystals on the supportsurface. The particle size is the same as the crystal size only when theseeding crystals exist as distinctive, individual particles. Inpractice, seeding crystals often exist as particles comprisingagglomerates of individual crystals. For example, a first solution cancomprise the 1.5 μm average seeding crystals and a second solution cancomprise 100 nm average seeding crystals. Preferably, the crystals usedas seeding crystal are pure in terms of both chemical composition andcrystal phase. Deposition of strange particulates or matters in the seedcoating would likely cause membrane defects.

The seeding solution can be applied onto one side (i.e., the front side)of the porous metal support sheet by spraying, dip coating, slip coatingor flow coating techniques. In any coating means, deposition ofexcessive coating solution on the support surface should be avoided. Aprotective layer can be applied to the opposite side of the support(i.e., the back side) to limit formation of the seeding layer to thefront side. An exemplary protective layer can include a fluoropolymermaterial, such as polytetrafluoroethylene (PTFE), that is stable in thecoating and growth solution. Generally, any material that can mask theback side to prevent accumulation of the seed coating solution on thebackside of the support sheet can be suitable.

In one example, the front side of the porous metal support sheetcontacts the seeding solution for a certain time typically ranging froma few seconds to one minute in spraying coating. The support sheetshould contact with the coating solution in uniform manner to obtain auniform coating. The other side of the support sheet is protected by acover to prevent its contact from the solution. The zeolite seedingcrystals are deposited on the support sheet surface by capillary forceupon contacting. If the spray time is too short, in-sufficient amountsof the seeding crystal is deposited. If the spray time is too long,excessive solution tends to accumulate on the support surface, which canresult non-uniform coating. The optimum solution/support contact time isdependent on the support sheet properties and solution properties. Forexample, a short time is needed if the seed loading in the solution ishigh and the support sheet is thin. After the support sheet/solutioncontact, the wet surface can be dried in ambient air, in an oven to dry,such as at 45 to 120° C., including 60 to 120° C. Multiple times of thespray coating may be conducted on the same support sheet using onesingle coating solution with intermittent drying to obtain uniform seedcoating at a suitable loading level.

In other examples, the substrate is coated two times with different seedcoating solutions. The first coating is made by using a solution havinglarger crystal sizes, while the second coating is made by using asolution having smaller size crystals. The sample is dried between thecoatings. Additional coatings can be applied, but are not preferable.The multiple coatings can result in a graded coating structure. In someembodiments, the average seeding layer thickness is not more than threetimes the size of the pore opening in the support sheet. For example,for pore sizes of 1 μm, the seeding layer thickness should be less than3 μm. Thicker coatings tend to result in delamination and/or mismatchbetween the zeolite layer and the metal support sheet.

After the seeding layer is dried, it is typically loose and does not yetcomprise an acceptable zeolite membrane because of the voids between thezeolite seeding crystals. Accordingly, the zeolite membrane layer iscompleted by forming the inter-crystal growth layer. Inter-crystalgrowth is conducted inside a hydrothermal reactor. The seeded metalsubstrate sheet is placed inside a pressure vessel and completelyimmersed inside the growth solution. The growth is conducted attemperatures from 60 to 160° C. for 0 to 12 hours. The growth solutionmust be homogenous without significant phase segregation when it isplaced still over one hour. The growth solution has suitablecompositions for growth of the target zeolite framework. For example,NaA-type membrane is grown on the NaA-type seeding crystals withNaA-type growth solution, Faujasite-type membrane is grown on theFaujasite-type seeding crystals with Faujasite-type growth solution, andMFI-type membrane is grown on the MFI-type seeding crystals withMFI-type growth solution. The reactor can be pressurized to a value thatkeeps the water in liquid phase.

During the inter-crystal layer growth, zeolite crystal growth fills upthe voids between the seeding crystals and bonds together, which resultsin a continuous, dense, strong layer. The dense, continuous zeolitemembrane layer is surprisingly free of pinholes and gaps. The zeolitemembrane layer leaves zeolite lattice channels as the only paths acrosswhich molecules can diffuse, thereby providing molecular sievingfunctionality. The hydrothermal growth may be repeated if some pin-holesremain.

The particular growth time and temperature can depend on the growthsolution compositions and the kind of zeolite membrane being prepared.Generally, the zeolite crystal growth rate increases with temperature,and crystal size grows with time. Generally, a shorter growth time isneeded with a higher growth temperature. However, the growth temperaturemay affect stability of the growth solution and/or support. For a givenzeolite framework, growth temperature and growth time can be balanced toobtain a quality membrane. Insufficient growth tends to leave some voidsand/or defects in the membrane. Over-growth can result in defectedmembranes and/or cause some side reactions. The defected membranes maycomprise cracks, delamination, and/or inclusion of large bulk particles.The zeolite growth is preferred to occur on the seed-coated supportsurface and not anywhere else such as in the bulk solution or inside thesupport pores. Thus, the growth conditions are controlled in a way thata thin, dense zeolite membrane film is formed.

After the hydrothermal growth, the residual solution and anyparticulates on the sheet are rinsed away with water and may be furtherwiped out. The wet membrane sheet is left in the fume hood for drying.The dried membrane may then be subjected to an appropriatepost-treatment. For example, if an organic template had been used in thegrowth solution such as in the case of MFI-type membrane growth, a heattreatment in oxygen/N2 or H2/nitrogen could be necessary to burn out theorganic template. The Na-form NaA or Faujasite membrane may be modifiedby exchange with other metal ions, such as Ag⁺¹, Li⁺¹, K⁺¹, Cs⁺¹, Ca⁺²,Mg⁺², Cu⁺², and La⁺³. Ion exchange is a convenient way to change thezeolite membrane framework without going through the whole synthesisprocess, and to tailor the zeolite channel sizes and/or surfacechemistry to meet specific application needs. The exterior surface ofas-synthesized membrane may be modified through surface reactions withsilanes and amine-functional groups to modify the chemistry andstructure of the exterior surface of the zeolite membrane.

III. Exemplary Embodiment 1 A. Introduction

Zeolites are made of in-expensive raw materials and possess some uniqueproperties, such as molecular sieving functions and stability.Particularly, the zeolite framework is stable in hydrocarbon or organicsolvent medium and at elevated temperatures. These are very desirableattributes as a membrane material. Thus, there has been a stronginterest to making zeolite membranes worldwide. The feasibility ofmaking zeolite membranes and their unique molecular-sieving functionswere demonstrated in late 1990s and early 2000s. The reported flux andselectivity of a zeolite membrane was one or two orders of magnitudehigher than the polymeric membranes for solvent dehydration. Mostlysmall ceramic disks and single-hole tubes were used as a support toelucidate membrane preparation and structures, and to demonstrate basicseparation process concepts. Such supporting materials served thoseresearch purposes well. For actual membrane product development,however, novel product designs and manufacturing processes are needed toproduce large membrane areas suitable for industrial applications at acost competitive with existing separation technologies.

Among various zeolite frameworks, 4A (NaA) zeolite membrane is the focusof this Example. It is contemplated that the disclosed membrane productdesign concepts and preparation methods discussed herein are applicableto other zeolite membranes. The A-type zeolite has a strong affinityspecific to water molecule. Three A-type zeolite materials of respectivepore sizes of 3A, 4A (NaA) and 5A have been widely used as an adsorbentin today's industrial drying processes, including natural gas andethanol fuel dehydration. In adsorption or absorption processes, thesaturated sorbent has to be periodically regenerated by varyingtemperature and/or pressure, and a significant amount of thermal energyneeds to be supplied to compensate the heat of desorption. In membraneseparation, water is continuously removed through the membrane withoutchanging pressure and temperature of the process stream. The membraneseparation is very desirable for drying and dehydration of processstreams with large volume flow rates. Efficient drying and dehydrationtechnologies are needed by many industries. Two noticeable applicationareas are air dehumidification for buildings and dewatering of ethanolfuels and bio-fuels in general. Air dehumidification is much needed toenhance air conditioning efficiency for buildings in hot and humidclimate.

The H2O-selective membrane of present invention can be used for removalof water molecules from a variety of process streams with proper processconfiguration. FIG. 4A shows a simple method for removal of watermolecules from a H2O-containing fluid under pressures, such aspressurized natural gas and air without recovery and usage of mechanicalpumps. As the H2O-containing stream flows over the membrane surface, thewater molecule permeates through the membrane driven by the pressuredifferential across the membrane and the permeated water vapor isdischarged into atmospheric air or environment.

FIG. 4B provides a method of water removal by use of purge gas withwater recovery. In the method shown in FIG. 4B, a purge gas may beintroduced to the permeate side of the membrane to sweep the permeatedwater vapor out of the membrane module. The sweep gas can be cooled downto condense the water and the purge gas is recycled after the condensedwater is separated out.

FIG. 4C shows a method with a liquid-phase sweep fluid. The sweep fluidcarrying permeated water molecule can be regenerated, such as byheating. The regenerated sweep fluid can be re-used, while the watervapor released from the regeneration may be condensed for recovery ofwater.

If the H2O-containing feed stream is at atmospheric pressure or at lowpressure, FIG. 4D shows a method to recover the water by use of vacuum,such as rough vacuum, in the permeate side of the membrane. Thepermeated water vapor is condensed into water, and the non-condensablegas is pumped out to environment by a vacuum pump. The vacuum pumpgenerates a lower pressure in the permeate side than the feed sidepressure.

If H2O vapor partial pressure in the feed stream is low, such as humidatmospheric air, a two-stage water removal and recovery method is shownin FIG. 4E. First, an intermediate compressor pull the permeated watervapor out the membrane module and compresses it a condensation pressure,typically at environment temperature. The condensed water is recovered,and non-condensable gas is discharged by use of a 2^(nd)-stage vacuumpump.

Ethanol fuel production in the US has experienced rapid growth since2000. Currently, ethanol is produced from corn and sugar canes. It isexpected that future growth will come from non-food grade, cellulosicbiomass. Ethanol content in the fermentation broth of cellulose could belower than what is obtained with corn. Thus, removal of water from moredilute fermentation broths demands energy-efficient dehydrationprocesses.

The NaA membrane supported on ceramic alumina tubes has beencommercialized for ethanol and solvent dehydration. Instead of singleholes, several holes can be made into one membrane tube to increase themembrane area packing density. However, the tubular membrane cost citedin the literature is viewed too high for widespread usage. Membrane areapacking density is another important issue to applications requiringlarge membrane areas.

The area packing density increases with decreasing the tube diameter.Exploratory studies of capillary inorganic membrane tubes have beenreported with an attempt to achieve dramatic enhancement of the membranearea packing density. The feasibility to deposit a quality NaA membraneon an alumina hollow fiber of 1.2 mm O.D×0.6 mm thickness was shown. Theceramic capillary tubes tend to be brittle. An alternative isdevelopment of ceramic monolithic membranes. In the monolithic membranebody, a number of small membrane channels (<1.0 mm) are embedded in asturdy, porous ceramic matrix so that making and packaging ofindividual, fragile capillary tubes is avoided, and manufacturingproductivity of the membrane can be enhanced at the same time. Themonolithic designs represent promising progress toward getting thesurface area packing density of inorganic membranes close to polymerichollow fiber membranes.

The attempt to make flat sheet zeolite membranes has been reported inthe literature using sintered porous metal plates and metal meshes as asupport. However, those porous metal supporting structures had roughpores and were too thick. The thick support is associated with highmetal material costs and mass transport resistance. The rough pore ofthe support requires thick coating of modification and/or membranelayer. The thick coating adds membrane preparation complexity andpresents potential adhesion/crack problems.

The disclosed thin-sheet zeolite membrane is designed to reduce membranefabrication cost and achieve high separation performances at the sametime. The disclosed zeolite membrane are as thin as metal foils andpapers that the membrane sheets can be manufactured with high throughputat competitive costs and packaged into membrane modules of area packingdensity as high as polymeric membrane sheets. This unique combination ofperformance attributes has not been obtained yet with conventionalceramic or polymeric materials alone.

i. Thin-Sheet Zeolite Membrane Design Concept and Fundamental Analysis

FIG. 1A provides an illustration of an exemplary membrane. As shown inFIG. 1A, a thin (<200 μm, typically between 20 to 200 μm) porous sheet10 serves as a support, on which a zeolite membrane layer 20 is directlydeposited. The support has a smooth surface and pores at micro andsub-micrometer levels, free of any large pores (>10 μm) for thepreparation of a continuous zeolite membrane layer. There is no chemicalbonding between the zeolite membrane layer and underneath support. Themembrane layer 20 adheres onto the support 10 mainly through mechanicalinterlock. There is a certain penetration of zeolite membrane growthinto the underneath support pores, such as greater than zero and lessthan 10 μm below the support surface, while the majority of the supportpores are intact (such as greater than 50% and up to 99%). The zeolitelayer provides desired molecular separation functions that allow certainmolecules go through while block other molecules and materials, whilethe support provides necessary mechanical strength of the membranesheet.

In addition to the surface pore structure, there are several otherfeatures to the disclosed support membrane. For example, the disclosedsupport sheet is so permeable that its transport resistance isinsignificant relative to the membrane layer, such as the permeancethrough the support is about an order of magnitude higher than thepermeance through the membrane layer. The support is stable during themembrane preparation and has long-term stability under the separationconditions. The zeolite membrane growth often involves usage of a strongbasic solution at elevated temperatures. Sometimes, the membrane issubject to thermal treatment after growth. The disclosed support ismechanically strong enough to withstand a pressure gradient (typically≥1 bar) under the operation conditions for a long time (a few years).Light weight of the support with area density <150 mg/cm² is also adesirable attribute. For example, 1-mm thick sintered steel and Tiplates with 40% porosity would have an area density of 265 and 471mg/cm2, respectively. By contrast, a 50 μm-thick Ni alloy sheet with thesame porosity has a surface density of only 26.7 mg/cm². Finally, thesupport sheet needs to be cost effective. The process disclosed hereinprovides a thin porous metal sheets that are suitable for preparation ofthe thin-sheet zeolite membranes which has all of these aforementioneddesirable properties.

A theoretical analysis was conducted to understand fundamentalfeasibility of the disclosed membrane structure. The stress isconsidered as one critical issue for fabrication of a supported zeolitemembrane, because of a large difference in physical and chemicalproperties between the support and membrane layer. There could bechemical stresses and thermal stresses. Some changes in the zeolitelattice structures can occur when different molecules and/or atoms areintroduced into the zeolite pore. The chemical stress is applicationspecific, while the thermal stress is commonly encountered. The metalsheet has high thermal expansion coefficient, while zeolite materialshave low or nearly zero thermal expansion coefficient. The thermalstress can be induced during the hydrothermal membrane preparationprocess, where the sheet needs to be heated and cooled between thegrowth and room temperature. The thermal stress can often occur duringapplication when the process temperature varies. Thus, theoreticalanalysis of the thermal stresses was performed to rationalize themembrane design and preparation.

In order to analyze the thermal stress in the zeolite membrane sheetconstrained by the support or substrate, the inventors developedphase-field models to mimic the actual membrane structure numerically byconstructing a 3-dimensional heterogeneous planar structure including afilm (zeolite membrane) and a porous substrate structure (porous Nisheets). An iteration method described below was employed to calculatethe thermal stresses in an elastically inhomogeneous material systemsubject to a temperature change.

ii. Construction of Zeolite Film on Porous Ni Substrate

The phase field (PF) method, as a unique mesoscale simulation tool, hasbeen successfully applied to predict complex three-dimensionalmicrostructure evolution kinetics in materials processes, such assolidification, ferroelectric and ferromagnetic phase transition,phase-separation and precipitation, martensitic transition, dislocationdynamics, twinning and de-twinning, and electrochemical processes. Thephase-field model (PFM) of spinodal decomposition is developed to createporous metal sheets. Two-phase equilibrium in a binary alloy isconsidered as an analogue in the PFM. When overall concentrations of thealloys fall into the spinonal decomposition region, a phase separationtakes place and two-phase microstructures are formed. The porousstructure can be obtained by assigning one of the two phases to bepores. By changing the model parameters, one can use PFM to generateporous structures of different pore size distribution and volumefraction. For example, the overall concentrations of the alloy willdetermine the volume fraction of two phases, and the coarsening time canbe used to control the pore size. Detailed description of the PFM methodcan be found in the literature (see, for example, Chen, Annual Review ofMaterials Research 32 (2002) 113-140 which is hereby incorporated byreference in its entirety). FIGS. 2A-2D present the porous structurecreated by the PFM, which have different pore sizes but the same volumefraction (50%). If the orientation of zeolite crystals in the membraneis not considered, i.e., the membrane is assumed to be uniform, thesupported membrane structure can be constructed by attaching a membranefilm onto the porous substrate structure. A uniform membrane film withcertain penetration depth into the porous support is illustrated in FIG.3. It is possible to create a polycrystalline zeolite membrane on theporous support by PFM of poly-crystal growth. In the present example,only uniform films were considered.

iii. Stress and Failure Analysis in Zeolite Films on Porous Ni Substrate

The geometry and coordinate system of the supported film structureconstructed for thermal stress analysis are shown in FIG. 3. Forsimplicity, it is assumed that the polycrystalline NaA film and nickelphase have homogeneous thermo-mechanical properties, which can becalculated with their properties of single crystals. The materialsproperties used in the simulations are listed in Table 1.

TABLE 1 Thermo-mechanical properties of NaA and Ni phases Single Lowerbound Upper bound crystal of poly- of poly- Aver- Material properties(GPa) crystalline crystalline age Elastic C₁₁ 171.3 167.3 167.4 167.4constants of C₁₂ 11.6 13.5 13.6 13.6 NaA C₄₄ 75.0 76.9 76.9 76.9 ElasticC₁₁ 248.3 284.4 306.0 295.2 constants of C₁₂ 152.2 134.2 123.3 128.8 NiC₄₄ 120.2 75.1 91.3 83.2 Thermal expansion coefficient α (NaA) −5.2 ×10⁻⁵/K  25~100° C.  5.0 × 10⁻⁵/K 100~150° C. −5.0 × 10⁻⁶/K 150~450° C.Thermal expansion coefficient of α (Ni) 1.5 × 10⁻⁵/K Fracture strengthof NaA 0.0026~0.026C₄₄

The abrupt change of thermal expansion coefficient with temperature isdue to the phase transition. In the simulations, this discontinuouschange was considered. The iteration method (Hu and Chen, Acta Mater.,49(2001)1879, which is hereby incorporated by reference in its entirety)was used to calculate the thermal stresses in an elasticallyinhomogeneous material system subject to a temperature change. With themodel, the effect of porous structures, membrane thickness, penetrationdepth, and temperatures on stresses and failure mechanisms of themembrane were investigated. Different stress states cause differentfailure mechanisms. The tensile stress may cause cracking, while thecompress stress on the thin film may cause buckling. σ₁, σ₂, σ₃ arenormal stresses, while σ₄, σ₅, σ₆ are shear stresses. In xyz coordinateshown in FIG. 3, σ_(xx)=σ₁, σ_(yy)=σ₂, σ_(zz)=σ₃, σ_(yz)=σ₄, σ_(xz)=σ₅,σ_(xy)=σ₆.

iv. Effect of Porous Structures

Four porous Ni substrates were used in the simulations. The porous Nisubstrates have the same pore volume fraction (50%), but different poresizes. The stress distributions on the cross-section of the membranestructure are calculated, i.e., plane A shown in FIG. 3. It is clearlyfound that the maximum tensile and compressive stresses locate near theinterface between the film and substrate. To manifest the effect ofporous structures, the probability distribution of stresses in a regionz₀−2dz≤z≤z₀+2dz was calculated where z₀ is the z-coordinate of theinterface between the film and substrate, and dz is the grid size. Theprobability distribution functions for stresses σ₁ and σ₃ are plotted inFIGS. 5A and 5B. The tensile stress σ₁ may cause the film cracking onthe plane. The probability of the maximum σ₁ value for the support ofthe smallest pore is higher than that for the other three supports oflarger pores. The probability of the maximum compressive stress σ₃,which may cause the interface cracking, decreases as the pore sizeincreases.

v. Effect of Film Thickness and Penetration Depth

The effects of film thickness and penetration depth on the stresses weresimulated with the porous support structure Case 3. FIG. 6 shows thestress distributions for the membrane films of the same penetrationdepth (H₁=4) but different film thickness. From the stress distributioncontour, we can see that the maximum tensile and compressive stressesweakly depend on the film thickness if the penetration depth is thesame. The conclusion was also confirmed by analyzing the probabilitydistributions of the stresses near the interface. The effects of filmpenetration depth on the stresses are shown in FIG. 7. The probabilitydistributions of the stresses near the interface are plotted in FIG. 8A.For comparison, the results in the case where the penetration depth iszero are also plotted in FIG. 8B with the symbol of black circle. Inthis case, the stresses are constant and the stress component σ₃ iszero. A clear trend can be observed that the maximum tensile andcompressive stresses increase with the penetration depth.

vi. Failure Mechanisms

Because the thermal expansion coefficient of zeolite strongly depends onthe temperature as presented in Table 1, the stresses and failure modesmay vary with the temperature. FIG. 9A plots the lattice mismatch strainvs the temperature. The mismatch strain is positive when the temperatureis lower than 100° C. while it is negative when the temperature ishigher than 100° C. For the film structure with film thickness (h₀=10),penetration depth (h₁=4) and porous structure (Case 3), the maximumtensile and compressive stress σ₁ and σ₃, the maximum shear stress σ₅ onthe interface are calculated for different temperatures. The results areplotted in FIGS. 9B-9D. Three different failure mechanisms areschematically drawn in FIG. 9E. The critical stresses for differentfailure modes are estimated from the data in the literature and listedin Table 1. Film cracking is determined by the maximum tensile stressσ₁. Film buckling is determined by the maximum compressive stress σ₁.When the maximum tensile stress σ₃ and shear stress σ₅ on the interfaceare larger than their critical values the interface cracking may occur.According to the critical stresses for different failure modes, thefailure modes are determined, and shown by the shadow regions in FIGS.9B-9D. At low and high temperatures, the failure mode could be interfacecracking because of the large tensile stress σ₃ and shear stress σ₅. Athigh temperatures, the large tensile stress σ₁ may cause the filmcracking, while at low temperature the large compressive σ₁ may lead tofilm buckling.

In summary, the simulations demonstrate that 1) the maximum thermalstresses are found near the interface between the film and substrate; 2)for a given penetration depth, the stresses weakly depends on the filmthickness; 3) penetration of film into the porous substrates causes themaximum stresses increase; 4) pore structure has stronger effect onstresses than penetration depth. The model enables one to investigatethe effects of film thickness, penetration depth, support porestructures, and temperature on the thermal stresses and failure. Themodel has not taken into account the defects such as de-coherence atinterface, micro-voids/pinholes, interface roughness, surface roughness,and grain boundaries. The calculated stress distributions can be used toassess potential cracking near defects.

B. Materials and Methods

The porous Ni support sheet was prepared in-house with a processdescribed in Liu and Canfield (J. Mem. Sci., 409-410 (2012)113-126),which is hereby incorporated by reference in its entirety. The porous Tisupport sheets were supplied by ADMA Products. Three NaA zeolite powdersused for preparation of seeding crystals are Nano-Zeolite LTA fromNanoSpace denoted as nano-A, synthetic Zeolite 4A (2-4 μm) from Wakodenoted as micro-A, synthetic Zeolite 4A (<75 μm) denoted as macro-A.The materials used for preparation of growth solution are NaOH (>97%),aluminum hydroxide, sodium silicate powder G (SiO₂:Na₂O=3.22), andde-ionized water.

Membrane Preparation.

Ball milling of the parent NaA powder as seeding crystals was conductedby rolling a mixture of 200 g H₂O, 10 g zeolite powder, and 100 beads ina 500-cm³ polypropylene bottle at 60 RPM. Attrition milling of NaApowder was conducted by mixing 50 g zeolite powder and 60 g water in anattrition miller at 720 RPM. The particle size was determined using aparticle size analyzer (Microtrac VSR). The milled slurry was dilutedwith deionized water to obtain a seed coating solution. The solidloading was typically 1-3 wt %.

Spray coating was used to lay down the seeding crystal on the supportsheet. The support sheet was mounted onto a wheel surface. As the wheelwas rotated at 40 RPM, the coating solution was sprayed on the wheelsurface via a Siphon Fed Flat Fan Pattern nozzle (Exair). Severalsupport sheets can be mounted on the wheel surface and be coated in onebatch run. Continuous rotation of the wheel enabled the support surfacebe sprayed many times in a given period of spraying. The spraying wasstopped when the support surface got fully wet. It typically took 1 minfor 50 μm-thick porous Ni support sheets. After the coated support sheetwas dried, the sheet was took off the wheel and weighed. If the seedloading was less than the target and/or the coating looked not uniform,the coating was repeated. With each coating solution, 2 or 3 times ofcoating were often conducted.

The growth solution was prepared in three steps. In first step, an Alprecursor solution was prepared by heating a mixture of 19.48 g Al(OH)₃, 35.14 g NaOH, and 360 g water H₂O in glass flask at 95° C. undercontinuous stirring for 2 hours. After the Al(OH)₃ powder was dissolvedand a clear solution was formed, the heating was stopped and thesolution was let to cool down to room temperature. In second step, a Siprecursor sol was prepared by mixing 24.8 g sodium silicate and 310.68 gwater in a beaker at 80° C. under stirring for 30 minutes. After a clearsolution was formed, the heating was stopped and the solution was let tocool down to room temperature. In third step, the Al solution was addedinto the Si solution under stirring to form a milky solution.

The membrane growth was conducted in a planar reactor built and designedin-house. Firstly, the seeded support sheet was mounted onto a supportframe and the frame was inserted into the reactor chamber in verticalorientation. Then, the growth solution was introduced to fill the growthchamber. Then, the reactor was closed and pressure tested. If there wasno leakage, the heater was turned on to raise the growth temperature to90° C. at 1° C./min. The holding time at 90° C. was typically 3.5 hoursunless specifically noted. Finally, the reactor was let to cool downnaturally. The growth solution was discharged from the bottom of thereactor, and the membrane support frame was taken out. The membranesurface was rinsed with deionized water and the residual solids on thesurface were wiped out with a clean room cloth. Then, the membrane sheetwas left in the fume hood for drying. By use of a support frame, fourmembrane sheets could be loaded into the reactor and grown in one batchrun.

The resulting membrane was inspected with any visual defects. Then, itwas checked with Red40-colored water. The colored water was dropped onthe membrane surface. After a while, the other side of the membranesheet was checked to see if any color leaked through. If yes, themembrane was determined of large defects. The membrane sheet without anycolor leakage was chosen for gas separation tests.

Membrane Structure Analysis.

Morphologies and pore structures were analyzed using scanning electronmicroscopy (SEM). A JEOL 5900 SEM was used for all the screening work,while detailed analysis was completed using a JEOL JSM-7600F. Imageswere collected at a working distance of 10-12 mm with an acceleratingvoltage of 20 kV. Both Secondary Electron (SEI) and Backscatter Electron(BSE) images were collected. SEI provides better topographicalinformation of a sample, while BSE reveals elemental information thatcan be useful for displaying material distribution in a sample. Inaddition, an EDS (Energy-dispersive X-ray Spectroscopy) was used forquantitative elemental analysis.

For detailed analysis of some selected membrane sheets, a polishedcross-section area was carefully prepared by fixing the sample in anepoxy mount and grinding the mount with a series of successively smallergrit sand papers. The mount was polished using a 1 μm diamond suspensionand a sub-micron colloidal silica suspension. After the specimen wascleaned in a Fiscione Plasma Cleaner, a carbon coating roughly 15 nmthick was applied to reduce charging under the electron beam. Theresulting sample stub was placed into the JEOL JSM-7600F for examinationat 20 kV gun voltage. Images were taken at a probe current of roughly 2nA and a focal distance of 6 mm, while a nominal 20 nA beam and a focaldistance of 15 mm were utilized for EDS data collection using an OxfordX-Max 80 Silicon Drift Detector. The data were analyzed using the OxfordInca Microanalysis Suite, version 4.12.

Membrane Separation Tests.

Molecular separation characteristics of the membrane were characterizedby separation tests with air, humid air, and humid CO₂/air mixture. Gasflows from respective gas cylinders were introduced into the feed sideof the membrane testing cell by use of mass flow controllers. In thetests with humid gas, de-ionized water was delivered by a syringe pumpand pre-vaporized prior to mixing with the feed gas stream. The feedside of the test cell was typically maintained under atmosphericpressure, while the permeate side of the cell was pulled vacuum. Thepermeated water vapor was scrubbed by use of a 3A molecular sieveadsorbent bed. The permeated water was determined based on the weightchange of the adsorbent bed. The residual gas discharged by the vacuumpump and swept by a helium gas stream was analyzed by an on-line massspectrometer (Accu Quad, Kurt J. Lesker Co.). The gas permeation ratewas determined based on the residual gas composition and the sweep gasflow rate.

Permeation rate (R_(i)) of non-condensable gases such as N₂ and O₂ wascalculated based on the sweep gas flow rate (n_(s)) and molar fraction(x_(i,s)) in the sweep gas by the following equation:R _(i) =x _(i,s) ·n _(s)

Permeation rate of water vapor (R_(H2O)) was determined based on boththe scrubbed amount and sweep gas composition as follows:

$R_{H\; 2O} = {\frac{\Delta\; n_{water}}{t} + {x_{{H\; 2\; O},S} \cdot n_{s}}}$

Δn_(water) is the moles of water collected in the adsorbent bed within atime period of t.

Permeance of individual molecule is calculated based on the permeationrate and partial pressure differential:

$P_{i} = \frac{R_{i}}{{S_{m} \cdot \Delta}\; p_{i}}$Δ p_(i) = p_(F) ⋅ x_(i) − p_(P) ⋅ y_(i)

H₂O/air separation factor was calculated based the gas compositions inthe feed (x_(i)) and permeate (y_(i)) side of the membrane cell:

$S_{ij} = \frac{\left( {y_{i}/y_{j}} \right)_{P}}{\left( {x_{i}/y_{j}} \right)_{f}}$

C. Results

The procedures and conditions, formulations, and raw materials weresystematically studied to obtain quality thin-sheet zeolite membranes ofconsistent performances. The membrane sheets prepared by use ofsecondary growth method showed higher quality and better performancesthan those prepared via direct growth. Among various preparation stepsin the secondary growth, seed coating was found to be the critical one.Impacts of the seed coating on the membrane performance will bediscussed below. The procedures and conditions developed based on thein-house porous Ni support sheet were applied to the membranepreparation on other support materials to show the importance of thesupport.

i. Supports Used for Preparation of Thin-Sheet Zeolite Membranes

The pore size of porous support sheets can be characterized with twocommon techniques: capillary flow and mercury porosimetry. Pore sizedistribution measured with these two techniques is compared in FIGS. 10Aand 10B for two in-house porous Ni sheets. The distribution curves ofthe pore size measured with the same technique look similar for the twosheets of different thicknesses (50 μm versus 100 μm). However, thedistribution profiles measured with the two different methods for thesame sheet differ significantly. The results reflect differentcharacteristics of the pore structure for a porous sheet.

The capillary flow is based on gas permeation through the sheet, whilethe mercury porosimetry involves intrusion of a liquid from the externalsurface of the sheet into the interior pores. To a large degree, thecapillary flow measurement characterizes all the pores through which thegas (or fluid) flows along the sheet thickness. By contrast, the mercuryextrusion captures characteristics of exterior pores of the sheet. Thepore sizes obtained with these measurements are tabulated in Table 2.

TABLE 2 Pore sizes of porous Ni sheets measured with two differentmethods Ni sheet name 50 μm 100 μm Capillary flow technique Bubble pointdiameter, μm 0.73 0.71 Mean flow pore diameter, μm 0.39 0.31 Mercuryporosimetry technique Pore diameter at peak position, μm 0.79 0.72Volume-based median pore size, μm 0.74 0.58The bubble point pore diameters are much larger than the mean porediameter. Thus, a number of small pores contributed to gas permeation inthe capillary flow measurement. The pore diameters at the distributionpeak position from mercury intrusion analysis are fairly close to thepore volume-averaged diameters, which indicate the narrow pore sizedistribution. In general, the pore sizes of the porous nickel sheets arein the sub-micrometer level. The porous nickel sheets show uniform poresizes on the exterior surface with a mean pore size ranged from 0.6 to0.90 μm, depending on specific thickness. Pertinent to the thin-sheetzeolite membrane preparation, the present authors think that the poresize distribution measured with the mercury porosimetry is more useful.

It was found that direct examination of the surface pore structures of asupport sheet with SEM is beneficial to membrane preparation. The poresize values obtained with the above techniques are all calculated basedon certain model equations from experimental data. SEM analysis revealsactual physical features of the membrane support surface. FIGS. 11A-11Dshow the SEM images of typical porous Ni sheets studied for zeolitemembrane preparation in this work. The three sheets of differentporosity exhibits uniformly porous surface structures. The 3-D networkedpore structure is revealed by the cross-sectional SEM image of the 50%porosity sheet (FIG. 11B). It is clearly seen that the surface poresvary significantly in both sizes and shapes. The surface porosity andpore size tend to decrease with decreasing sheet porosity, whichindicates uniformity of porous structures along the sheet thickness.

ii. Membrane Preparation on Thin-Porous Metal Sheets with Graded SeedCoatings

In the secondary growth, zeolite crystals of the parent zeoliteframework are first coated on the support to serve as a seed for crystalgrowth later. Thus, kinds of the seeding crystal and coating textureshave direct impacts on the subsequent zeolite membrane growth. To letthe zeolite crystals inter-grow to form a dense membrane layer, uniformseed coating of the support is necessary. In other aspect, some seedingcrystals need to be planted inside the support pores to let certainpenetration of the zeolite membrane growth into underneath support poresoccur to enhance the membrane adhesion.

Sequential coatings of the support with different sizes of seedingcrystals were chosen first to obtain the desirable seed coatingstructures. The idea was to use a larger seeding crystal to cover thelarge support pore and smoothen the seed coating by use of a smallerseeding crystal. The larger seeding crystal was prepared by ball-millingof parent NaA powder with product specification of 2-4 μm particle size,which is named as micro-A in this paper. The smaller seeding crystal wasprepared by ball-milling of parent NaA powder with product specificationof 300 nm nano-zeolite, which is called as nano-A. The coating solutionwas prepared in de-ionized water with 1 wt % solid loading. FIGS. 12Aand 12B show particle size distributions for the two coating solutions.The particle size distribution for nano-A is very broad with d50=1.57 μmand D95=13.2 μm. This seems to be contrary to what would have beenexpected for such dilute slurry made of nano-sized parent crystals. Thesmall crystal size of this sample was confirmed by XRD and SEM analyses.The broad distribution is mainly due to agglomeration. The smallcrystals have high surface free energy and tend to cluster together. Themicro-A seeding solution has d50=1.63 μm and D95=3.97 μm, which iswithin the product specification. It seems that the particle size wasnot reduced by the ball-milling. The discrepancy between the actualcrystal size and measured particle size is also explained byagglomeration of the crystals.

The porous Ni sheet was spray coated with micro-A first and followedwith spray coating of nano-A. The seed loading is characterized bysurface loading density as calculated below:

$\omega_{s} = \frac{W_{s} - W_{0}}{{SA}_{s}}$

The nominal coating thickness can be estimated based on the surfaceloading density by assuming a uniform layer of coating:

$\delta_{s} = {\frac{\omega_{s}}{\rho_{s}} \times 10}$

Similarly, the membrane growth surface density and nominal membranethickness can be calculated with the following equations:

$\omega_{m} = \frac{W_{m} - W_{0}}{{SA}_{m}}$$\delta_{m} = {\frac{\omega_{m}}{\rho_{m}} \times 10}$

First set of membrane preparation studies was conducted by varying theseed loading while keeping the membrane growth conditions constant. Thepreparation results are summarized in Table 3.

TABLE 3 Membrane preparation with sequential coatings of two differentparent zeolite powders as seeding crystals micro-A nano-A Total GrowthThickness Mem ID Support seeding seeding seeding Growth gain from SEM61509- porosity % mg/cm² mg/cm² mg/cm² time h mg/cm² μm 27-26-1 38.40.011 0.045 0.056 3.5 1.7 4.8 9-7-4 30.4 0.070 0.000 0.070 3.5 1.6 7.4to 8.1 27-26-2 38.4 0.085 0.055 0.140 3.5 1.7 4.4 to 7.3 27-26-4 35.50.123 0.155 0.278 3.5 2.0 5.3 to 8.4 27-26-3 33.8 0.147 0.212 0.358 3.52.2 9.3 28-26-2 37.7 0.074 0.075 0.149 1 1.7 4.4 27-26-2 38.4 0.0850.055 0.140 3.5 1.7 4.4 to 7.3 28-26-1 36.3 0.070 0.061 0.131 6 2.1 6.9As the seed loading density was varied dramatically from 0.056 to 0.358mg/cm², the growth density only increased slightly from 1.7 to 2.2mg/cm². There was no strong correlation between the seeding density andmembrane growth density. With a packing density of 1 g/cm³ for the seedcoating, the nominal coating thickness is from 0.56 to 3.58 μm, which iswithin the range of the expectation. With a packing density of 1.6 g/cm³for the membrane layer, the calculated nominal membrane thickness isfrom 10 to 14 μm, which is fairly thick. In the second group of studies,the supported sheets of similar seeding density were used while theholding time at 90° C. growth temperature was varied. Significant weightgain was obtained even after 1-h growth hold. Further increasing theholding time to 6 hours did not result in substantial weight gain.

Desirable structural features were revealed by SEM analysis. The surfacetexture after seed coatings are illustrated by the SEM images in FIGS.13A-13D. After spray coating with the micro-A seeding crystals, the baresupport remained visible but the support surface was uniformly decoratedwith the micro-A seeding crystals (FIG. 13A). Under high magnification(FIG. 13B), small fragments of the seeding crystal can be seen clearly.After further spray coating with the nano-A seeding crystal, the supportsurface was substantially covered (FIG. 13C). Some support poresremained visible but were significantly narrowed compared to the baresupport. Presence of small or nano-sized seeding crystals is evidentunder high magnification (FIG. 13E).

The membrane surface textures looked all the same for these membraneslisted in Table 3, which are illustrated in FIGS. 14A and 14B.

The surface uniformity can be seen from the low-magnification picture.The crystal structure of the zeolite membrane is clearly seen under thehigh magnification. The membrane surface is not very smooth butcomprises densely inter-grown zeolite crystals. Some large crystals onthe surface were likely deposited from the bulk phase during growth. EDSanalysis of several spots on the membrane shows consistent atomiccomposition, which is a good indication to crystal purity of theresulting membrane.

The membrane design features are well revealed by SEM analysis ofpolished cross-section of the membrane sheet. The images for thosemembranes grown under the same conditions on the supports of differentseed loadings are compared in FIGS. 15A1-15E2. First of all, themembrane thickness is much less than the value calculated based on thegrowth weight gain. There is some correlation between the seed loadingand membrane thickness measured from the SEM image. The membranethickness increases with the seed loading. However, the membranethickness is not uniform throughout the membrane sheet and varies withlocation. Regardless the thickness, all the membranes showed a densemembrane layer. The some cracks shown in the SEM images could be causedduring the SEM sample preparation, while defected membrane spots werecertainly observed. As shown by FIG. 15C2, a void exists in the membranelayer. Fortunately, the void is surrounded by a dense zeolite membraneshell. This phenomenon indicates that the secondary growth has abilitiesto repair some defects on the seeded sheet. The large difference in themembrane thickness between the actual measurement and calculation isexplained by deposition of some materials inside the support pores,which can be seen from the SEM image. Inevitably, some growth solutionwould get into the support pores at the beginning of growth. Aftergrowth, the trapped growth solution in those pores would leave a soliddeposit. The cross-section images show that the majority of supportpores are intact despite the extra solid deposition.

The growth of membrane layer into the underneath support pore is shownby all these membranes as expected from the design. It can be seen thatthe support of higher porosity would provide more surface locations tohave mechanical interlocking. The cross-sectional images for the twomembrane samples grown with different holding time show the featuressimilar to the membrane grown at 3.5 hours. The results indicate a rapidzeolite crystal growth rate with the growth solution prepared in thisExample.

Molecular sieving functions of the resulting membranes are characterizedby air dehumidification tests. The results are summarized in Table 4.

TABLE 4 Impacts of seeding crystal loading and growth time on membraneperformances (testing conditions: 30.3° C., 3.1 mol % H₂O in feed air atatmospheric pressure) Air dehumidification performance Permeate, H₂Opermeance H₂O/N₂ separation Mem ID mbar mole/(m² · s · Pa) factor27-26-1 7 3.8E−06 11  9-7-4* 5 5.2E−06 364 27-26-2 6 5.9E−06 252 27-26-45 2.8E−06 321 27-26-3 5 3.2E−06 371 28-26-2 5 5.8E−06 485 27-26-2 65.9E−06 252 28-26-1 5 6.1E−06 331Except for first sample, all other membranes exhibit excellent H₂O/N₂selectivity. The seed loading density for first sample was likely toolow to have sufficient coverage of the bare support for growth of acontinuous dense membrane. Although no obvious defects were seen fromthe SEM analyses, some minor defects could have existed in thismembrane. Thus, the seed loading needs to be above 0.056 mg/cm². If theseed loading is too high, such as ≥0.278 mg/cm₂, H₂O permeance can bedecreased. There appears to be an optimum seed loading. It isinteresting to note that the H₂O permeance is similar for the threemembranes grown with different holding times on the support of similarseed loading (0.13-0.15 mg/cm²). It is confirmed that 1-hour growth timecould be sufficient, and prolonging the growth after the membrane isformed may not have any benefit.

iii. Membrane Prepared with One Kind of Seeding Crystal

The nano-A and micro-A are specialty NaA parent powder. The sequentialcoatings with these two materials were considered complicated. Thus,cheaper NaA powder materials and simplified seed coating processes werestudied to lower potential manufacturing costs of the seed coatingprocedure. The macro-A powder—a commodity material was identified to bea fairly effective seeding crystal for membrane growth. As-receivedpowder had rough particle sizes in tens of am and the particle quicklysettled when the powder was added into water. Thus, its particle sizehas to be broken down to obtain a stable coating solution. Attritionmilling was found to be a more efficient technique than ball-milling.FIG. 16 shows that 30-min attrition milling resulted in a particle sizedistribution profile similar to the one obtained with 43-hour ballmilling with a mean particle size of 1.7 μm. The mean particle size canbe readily reduced to 1.3 and 1.0 μm by increasing attrition millingtime. However, increasing the milling time caused tailing of theparticle size distribution, i.e., an increased particle size at 90%pass. An explanation for this is agglomeration of smaller fragmentsproduced by the milling. A relatively stable coating solution wasprepared with the milled powder for spray coating. Table 5 summarizesthe preparation and air dehumidification testing results of a group ofmembranes grown on the support seeded with one kind of coating solutiononly.

TABLE 5 Preparation conditions and air dehumidification performances ofzeolite membranes grown with only one kind of seeding crystal (airdehumidification testing conditions: 30.3° C., 3.1 mol % H₂O in feedair) Membrane preparation Air dehumidification Mem No of Seed GrowthH₂O/N₂ ID Support spray loading, gain, H₂O separation 61509 porosity %Seeding crystal coating mg/cm² mg/cm² p, mbar permeance factor 65-3 46.043-h ball-milled 3 0.19 1.18 5 7.6E−06 621 1.7 μm micro-A 65-2 42.7 43-hball-milled 1 0.20 2.04 14 8.7E−06 5 65-1 43.8 1.7 μm macro-A 2 0.251.73 5 7.1E−06 176 65-4 32.9 attrition-milled 3 0.21 1.51 5 4.8E−06 2801.3 μm macro-A 13-6 42.7 attrition-milled 2 0.17 1.91 5 1.2E−05 infinite11-5 42.4 1.1 μm macro-A 2 0.18 1.87 5 1.0E−05 3333

The membrane grown with the micro-A seeding crystal showed both higherH₂O permeance and H₂O/N₂ selectivity than those membranes with thesequential seeding of micro-A and nano-A. Thus, additional seed coatingwith nano-A is not necessary to obtain a quality zeolite membrane.One-time spray coating with the 1.7 μm macro-A seeding crystal resultedin a membrane of poor H₂O/N₂ selectivity, indicating that one-timecoating was not sufficient to disperse the seeding crystals uniformly onthe support surface. The H₂O/air selectivity was substantially enhancedby conducting two times of spray coating with the same seeding solution.SEM analyses of the seeded supports confirmed that the seeding crystalswere indeed better dispersed with two-time coating than one-time (FIG.17C versus FIG. 17B). The H₂O/N₂ selectivity was improved using the 1.3μm macro-A seeding crystal (mem #65-4 in Table 5). The lower H₂Opermeance for this sample is attributed to the low support porosity ofthe support. Excellent air dehumidification performances were obtainedwith the membranes grown by using the 1.1 μm macro-A seeding crystals ona support of about 40% porosity. H₂O permeance reached above 1.0E-5mole/s/m²/Pa with H₂O/N₂ separation factor above 3000.

SEM analyses of the seeded support (FIGS. 17A-17D) revealed nosubstantial difference in the composition and crystal size between themicro-A and macro-A after milling. The particle and crystal size of theparent NaA powder is not an issue for making an effective seedingcrystal. Instead, crystal purity was found to be important as seedingcrystals. For a given parent NaA powder, milling conditions can havesubstantial impacts on the seed coating and zeolite membrane growth. Theseeding crystals need to maintain crystal purity and have a suitablesize distribution profile so that some seeding crystals can get into thesupport pore while majority of the seeding crystals decorate exteriorsurface of the support. The SEM images showed presence of some isolatedlarger crystals on the seeded surface, indicating that additionaloptimization of seed coating may be desirable.

Both surfaces and cross-sections of this group of membranes wereanalyzed by SEM to reveal the membrane structural features. The surfacestructures for all these membranes looked the same or similar. Somecharacteristics between different membranes were shown by thecross-sectional analysis. Typical SEM cross-sectional images of the fourmembranes grown with the macro-A seeding crystals are compared in FIGS.18A-18D. With the 1.7 μm macro-A seeding crystal, the membrane does notappear to be locked into the support well (FIG. 18A). The membrane seemsto exist as a separate layer from the support. This is explained byin-sufficient penetration of the seeding crystal into support pores andin-sufficient zeolite membrane growth into the pore. The impact of thesupport porosity on the membrane growth is illustrated by FIG. 18B. Thesupport used for preparation of this membrane (mem #65-4) happened tohave fairly low surface porosity. The membrane seems to exist as asegregated layer. The low porosity caused blockage of permeation pathsafter molecules diffuse through the membrane layer, which explains thelow H₂O permeance of this membrane. By use of the support of higherporosity and macro-A seeding crystals of smaller sizes, excellentinterlocking between the membrane layer and support is shown (FIGS. 18Cand 18D). The cracks in FIG. 18D were caused by polishing during the SEMsample preparation.

It is found by this study that a support sheet of 35 to 45% porosityproduces consistently high H₂O permeance and H₂O/N₂ selectivity. Theoptimum particles size to use the macro-A as a seeding crystal isbetween 1.0 to 1.4 μm (d50).

The NaA crystal phase (Na₁₂(Si₁₂Al₁₂O₄₈)×H₂O) of the resulting membraneswas confirmed by XRD analyses. FIG. 19 shows that the XRD patterns offour membranes grown with different seeding crystals completely overlap.Thus, the zeolite membrane crystal phase is not affected by usage ofdifferent seeding crystals of different sizes. The membranes of the sameXRD pattern can exhibit very different gas separation performances. Thegas separation performances such as permeance and selectivity aresubstantially affected by detailed membrane structures that depend onmembrane preparation.

Another group of membranes (Table 6) were prepared using the same seedcoating process but different growth temperatures to further understandmembrane growth with the macro-A seeding crystal.

TABLE 6 Membranes prepared with standardized seed coating procedures andconditions (seeding crystal: attrition-milled macro-A; growthconditions: 1° C./min to 90° C.; separation testing conditions: 30° C.,3.3% H₂O in CO₂/air mixture) Membrane preparation Mem Seed Growth IDSupport loading Hold time gain Gas permeance, mole/m₂/s/Pa 60037-porosity % mg/cm² at 90° C. h Spent soln mg/cm² H₂O CO₂ O₂ N₂ 94-1 41.40.25 3.5 Solids settled 2.29 5.8E−06 ND ND ND quickly 96-94-1 44.1 0.241 some solids 1.89 5.7E−06 2.0E−09 ND ND settled but some remaineddispersed 98-94-1 40.3 0.22 0 No solid 1.35 settlingThe three porous Ni sheets of desirable porosity were coated with the1.1 μm macro-A seeding solution in one batch under the same conditions.The loading levels for the three sheets are approximately same—aroundabout 0.23 mg/cm². The zeolite membrane growth was conducted withdifferent holding times at 90° C. The weight gain after growth decreasedwith reducing holding time. The NaA crystal phase was identified by XRDanalysis for all the three membranes (FIG. 20). It is very interestingto note that the zeolite membrane can be formed at such a short holdingtime, indicating a rapid zeolite crystal growth rate on this macro-Aseed. The relative XRD peak intensity for the membrane grown with 0-hourhold matches well with that for the membrane grown with 1-hour hold.Thus, there is similar crystal orientation in these two membranes.However, the relative XRD peak intensity for the membrane grown with3.5-hour hold looks very different from the previous two. This can beexplained by deposition of zeolite crystals from bulk solution onto themembrane surface during growth. Extensive surface coverage of themembrane grown with 3.5-hour hold is shown by the SEM image in FIGS.21A-21F. The membrane surface textures revealed by SEM are consistentwith observation of the spent growth solution. The growth solutiongenerated from 3.5-hour holding showed rapid solid precipitation afterdischarged from the reactor. By contrast, the growth solution dischargedfrom the 0-hour hold growth was homogenous without any solidprecipitation or phase segregation, which looked like the fresh growthsolution. The solution discharged from 1-hour hold growth had a phasestate between the above two. Thus, the bulk phase zeolite growthoccurred significantly with 3.5-hour hold at 90° C. and large zeolitecrystals were formed. Those large crystals could adhere onto themembrane surface during settling under the growth condition. This studyshows that the zeolite membrane grows faster than the bulk-phase crystalgrowth, and the growth time can be optimized to obtain a dense membranelayer while minimize deposition/inclusion of the bulk-phase crystal onthe membrane.

The air dehumidification testing results suggest that deposition of thebulk crystal on the membrane surface may not affect H₂O permeance andH₂O/N₂ selectivity. But, inclusion of large bulk crystals into themembrane may induce some stresses and become a membrane durability issuethat remains to be studied in the future.

Inter-growth of the zeolite membrane layer with the support is clearlymanifested by these three membrane samples of adequate support porosityand seed coating. The SEM images of cross-sections in FIGS. 21A-21F showclear penetration of the membrane growth into underneath support poreswhile bulk support pores are fully open. The membrane structuralparameters for the thin-sheet zeolite membrane design are measured withthe SEM images and summarized in Table 7.

TABLE 7 Membrane/support interface features of the membranes preparedwith standardized procedures Mem ID 60037- Mem thickness Interface spot1 2 3 4 5 6 Max Avg 94-1 ~8.7 μm Pore width, μm 1.5 1.8 2.5 3.0 1.2 3.02.0 Depth, μm 3.6 1.9 2.3 2.3 0.9 3.6 2.2 96-94-1 8.21 to 8.63 μm Porewidth, μm 3.9 4.5 2.5 5.1 5.1 4.0 Depth, μm 1.2 5.4 2.3 2.3 5.4 2.898-94-1 ~4.9 μm Pore width, μm 2.0 3.0 4.0 1.7 3.1 1.3 4.0 2.5 Depth, μm1.3 1.0 1.2 2.1 2.5 1.6 2.5 1.6The membrane layer thickness can be as high as 8 to 9 μm without cracks.At such a membrane layer thickness, the zeolite membrane sheet exhibitsthe rigidity and flexibility same as the original metal support sheet.FIG. 22 shows that a 12 cm×21 cm zeolite membrane sheet can beself-stood and is also flexible to bend. The sheet can be bent onto ½″tube without cracking and is restored back to a flat sheet afterbending. This may be attributed excellent elasticity of zeoliteframeworks.

The support pore width and growth penetration depth are measured atdifferent interface spots for the three membranes. These measurementsare conducted at the spots of relatively large pore width to assess theupper limit of the support pore size and penetration depth for making astable and quality zeolite membrane with the present design idea andpreparation method. The smaller support pores are not a concern forformation of a continuous membrane layer and smaller penetration depthis not a concern for the membrane stress. Although no pore sizes greaterthan 2 μm were measured by the mercury porosimetry for these porous Nisupport sheets used, the SEM images revealed frequent presence of poreswith >2 μm width in these three membranes. The pore width andpenetration depth vary significantly at different spots. The averagevalues are mostly for reference purpose. For actual membranepreparation, the maximum allowable pore width and penetration depth aremore useful. Thus, the largest pore width and penetration depth from alimited number of spots are identified for each membrane in Table 7. Themaximum pore width looks to be about 5.1 μm, while the maximumpenetration depth is about 5.4 μm.

iv. Membrane Preparation on Supports of Rough Pores

To further understand the support pore requirements for direct growth ofa stable and quality zeolite membrane, the membrane preparation wasconducted with a few support sheets different from the porous Ni sheetused in all the above studies. The seed coating solution, spray coatingprocedure, growth solution, and growth procedure and conditions werekept to be the same. Table 8 lists the preparation conditions with fourporous Ti support sheets and air dehumidification testing results ofresulting membranes.

TABLE 8 Growth of NaA membrane on different support materials (standardseeding, growth solution, and growth procedures) Membrane Seed coatingGrowth Membrane Support load, thickness, gain, Leak Air dehumidificationtests ID Name Porosity mg/cm² um mg/cm² test P, mar P_(H2O) H₂O/N₂127-124-1  50 μm-Ni 0.32 0.20 1.71 1.34 No 5 8.0E−6 347 127-124-6 134μm-Ti 0.37 0.18 1.49 4.56 leak 127-124-11 0.37 0.31 2.61 3.68 leak127-124-7 150 μm-Ti 0.44 0.16 1.33 4.79 leak 127-124-12 0.44 0.28 2.353.74 leak 127-124-8 43 μm/ 0.47 0.17 1.41 4.54 minor 11 4.7E−6 4 250 μmTi leak 127-124-13 0.47 0.28 2.32 3.03 leak 127-124-14 43 μm/ 0.57 0.282.36 3.71 No 9 6.1E−6 11 127-124-9 140 μm Ti 0.57 0.38 3.19 4.50 No 73.5E−6 10A porous Ni sheet was used in this batch of studies for control purpose.The two porous Ti sheets have a symmetrical structure—same as the porousNi sheet, while the other two Ti sheets have an asymmetric porestructure. For each Ti support sheet, two seed loading levels were usedfor membrane growth. The rationale was that those support sheets oflarger pores than the porous Ni sheet would need more seed coatingmaterial to cover. The growth weight gains for those Ti support sheetsare about 2 to 4 times of that for the Ni sheet. However, except for thelast two membranes, all the other Ti-supported membranes leaked. Theleakage was checked by dropping Red40-colored water on the membranesurface. The color molecules would emerge in the other side of themembrane sheet if there are major defects on the membrane. For a qualityzeolite membrane such as the Ni-supported one, the color moleculepermeation would be blocked. It is noted that for the first three Tisupport sheets, more weight was gained from growth with the support ofless seed loading. It seems to be contrary to general perception. Theproblem was caused by more zeolite growth in interior pores of thesupport of less seed loading, because a continuous membrane was notformed and the solute in the growth solution could consistently diffuseinto the support pore during growth process. The membranes grown on the43 μm/140 μm Ti support did not show color leakage, which is anencouraging sign. However, air dehumidification tests showed that H₂ON₂separation factor is only 10-11, which is dramatically less than 347 forthe Ni-support membrane made in the same batch. For this support,increasing the seed loading did result in higher growth weight gain. Thepermeance decreased but the selectivity was not improved.

The root causes are revealed by SEM analysis. FIGS. 23A-26C show SEMimages of the bare support, after seeding, and after growth for the fourTi supports, respectively. FIG. 23A shows that majority of the 134 μm-Tisupport surface comprises pores of micro and sub-micrometer level.However, presence of pores as large as 10×20 μm was found. These largepores still exist after seed coating (FIG. 23B), because the pores aretoo large to be covered. Presence of large pores at tens of μm level wasalso found for the 150 μm-Ti support (FIGS. 24A-24D). Those large poresremained open after seed coating. Increasing the seed coating from 0.16to 0.28 mg/cm² did not help. After zeolite membrane growth, those largepores were narrowed down significantly. However, the holes remainedthere. The pores were too large to be closed by the secondary crystalgrowth.

The coated Ti sheet (43 μm/250 μm Ti in FIGS. 25A and 25B) showed afairly smooth surface with average pores at micrometer level.Unfortunately, there were holes of 10 to 20 μm size, and such holes aretoo large to be covered by the seed coating and secondary growth. Thecoating-modified 43 μm/140 μm Ti sheet exhibited the smoothest surfaceamong the four Ti sheets studied (FIGS. 26A-26C). The average pore sizefor this support is much smaller than the first two and is comparable tothe third Ti. Large holes at tens of μm level were not found. However,some surface pores or cavities of as wide as 20 μm still existed. Afterseed coating, majority of the support surface was covered by the seedingcrystals. However, there were pores of width at about 5 to 10 μm level.After the zeolite growth, majority of the surface was covered with adense inter-grown crystal layer. But, some cavities remained. Thismembrane showed molecular sieving functions but there must be largedefected pores that limit the selectivity.

It becomes clear through this group of comparative membrane preparationstudies that the support surfaces have to be free of large holes, pores,and cavities for making a thin-sheet membrane of high permeance, highselectivity, and good adhesion. Judging from the mean pore size and evenfrom pore size distribution is in-sufficient, because presence of someisolated large pores and/or holes can ruin the membrane. Directexamination of the support surface structure is necessary. Forpreparation of thin-sheet zeolite membranes with the design principleand preparation method presented in this paper, the support surface needto be free of pores or defects above 10 μm.

D. Conclusion

Thermal stress distribution in the zeolite/metal sheet membrane has beenmodeled to understand impacts of some important structural and materialparameters (membrane layer thickness, penetration depth, support poresize, thermal expansion coefficient) on formation of potential membranedefects. The thin-sheet membrane design concept is also elaboratedexperimentally using secondary growth method. The theoretical stressmodeling results qualitatively agree with the experimental findings.

Seed coating is one critical step in making a high-performance membrane.The seeding crystals need to be uniformly dispersed on the supportsurface for uniform membrane growth, and a fraction of seeding crystalsalso need to be deposited into the outer pores of the support forcertain penetration of membrane growth into underneath support pores. Itis found that parent NaA powder of conventional sizes after propermilling can act as an effective seeding crystal. Spray coating isdeveloped as a simple method to lay down the seeding crystals on thesupport sheet. An appropriate seed loading surface density (mg/cm²) isnecessary to obtain a quality membrane. The membrane layer thickness <10μm and penetration depth <5.5 μm were found with the membranes that arestable exhibiting both high permeance and selectivity.

The support surface textures are vital for formation of a continuous,dense zeolite membrane layer. In the present example, knowing the meanpore size is not sufficient. Direct examination of a support surface bymicroscopy allows one to assess if it is suitable for making theproposed membrane. The support surface needs to be substantially free ofany holes, pores, and cavities above 10 μm, i.e., the probability tofind such large defects under microscopy is less than 1% ifrepresentative given membrane support surfaces are sampled. Thepreferred support porosity is from 20 to 50%. H₂O permeance of 1.0E-5mole/s/m²/Pa with H₂O/N₂ separation factor above 3000 can be obtainedwith an optimum membrane.

IV. Exemplary Embodiment #2

In this specific example, the template-free solution for thewater-selective zeolite membrane, NaA, has a composition comprising4Na:2Si:2Al: 150H₂O. It is prepared by forming an aluminate solution bydissolving 7.95 g sodium hydroxide (Wako, >97%) and 10.82 g aluminumhydroxide (Wako, >95%) in 45 g D.I. water by stirring at 100° C.(temperature measured by thermometer) for 30 minutes to get a non-clearsolution. A silicate solution was prepared by dissolving 13.78 g sodiumsilicate (Wako, 17-23% Na₂O) in 123 g D.I. water by stirring at 50° C.for 2 hours to get a clear solution. The aluminate solution was thenadded into the silicalite solution drop-wise at room temperature and theresulting mixture was stirred vigorously for 30 minutes at roomtemperature to produce a homogeneous gel without observableprecipitation or phase segregation. The preferred conditions forinter-crystal layer growth using the above solution are about 3 hoursand 100° C.

FIGS. 27A-27C are scanning electron microscope (SEM) micrographs showingthe structure of the water-selective (NaA) membrane prepared byhydrothermal growth of seeded porous metal substrate with template-freesynthesis solution, as described above. FIG. 27A shows the surface ofthe porous metal substrate coated with seed crystals having a size ofabout 0.3 μm. FIG. 27B shows the surface of the membrane layer afterhydrothermal growth. FIG. 27C is a cross-sectional view of the membranesheet. The pores of the metal support 101 are fully covered by two timesof coating with NaA seed crystals of about 0.3 um crystal sizes.Distinctive seed crystals are visible on the seeded substrate. A denseand continuous zeolite membrane 102 is formed after hydrothermal growth.The cross-sectional view of fractured membrane sheet shows certainpenetration 103 of the seed crystal into the support pore, which isimportant to have strong adhesion.

For forming a hydrocarbon-selective or an alcohol-selective membrane, asimilar procedure is followed except the seeding and inter-crystalgrowth solutions would contain precursors for the hydrocarbon-selectiveor the alcohol-selective zeolite and, in the particular example below, atemplate-containing synthesis solution was used. For example, asilicalite membrane can be prepared according to the following. A thinporous Ni sheet is first spin-coated with approximately 100 nmsilicalite seed crystals. The seeding suspension was prepared by mixing10 g of 3 wt % silicalite suspension with 4 g of 20 wt % polyethyleneglycol (PEG) solution and 5 g of 25 wt % surfactant C18EO10/isopropanolsolution under sonication. The suspension was shaken for two days. Theseeding suspension was degassed and filtered using a 5 μm filter beforespin coating. The thin porous Ni sheet was placed on a magnet tape andthe spin-coating was conducted using a spin coater. The spin rate was1500 rpm. After that, the seed-coated substrate was dried at 150° C. for20 minutes.

The silicalite synthesis solution for hydrothermal growth had a molarcomposition of 1.0 TPAOH: 8.14 TEOS: 589H2O and was prepared by mixing5.65 ml Tetrapropylammonium hydroxide (TPAOH, 1M solution in water,Aldrich), 10.2 ml Tetraethyl Orthosilicate (TEOS, 98%, Acros) and 60 mlD.I. water. The mixture was stirred at 5° C. for 2 hours and a clearsolution was obtained. A disc substrate was mounted in a reactor withits seeded surface facing down-wards. The reactor was put into an ovenpre-heated to 180° C. for 2 hours and then cooled down naturally in air.The sample was taken out, rinsed under running D.I. water, dried withcompressed air, and kept in a 50° C. oven overnight. The silicalitemembrane sheet was then heated at 400° C. for 4 hours in 2 vol. % O2/N2mixture. The rate for both heating and cooling were 1° C./min.

FIGS. 28A-28D are SEM micrographs obtained from a hydrocarbon-selectiveand/or alcohol-selective zeolite membrane formed according toembodiments of the present invention. FIG. 28A shows the porous metalsubstrate coated with silicalite seed crystals. FIG. 28B shows thesurface of the membrane layer after hydrothermal growth and calcination.FIG. 28C is a cross-sectional view of the membrane sheet. FIG. 28D isanother cross-sectional view showing a smaller scale with greater detailand, again, shows certain penetration 203 of the seed crystal into thesupport pore. The membrane was prepared by hydrothermal growth of theseeded substrate with a template-containing synthesis solution asdescribed above. Referring to FIG. 28C, a continuous zeolite film 201 ofthickness 1˜2 um is formed on the substrate 202. Referring to the X-rayDiffaction (XRD) spectrum in FIG. 28E, existence of the zeolite crystalstructure is verified by the thin-film XRD measurement.

In some instances, the zeolite membrane layer can be formed directly onthe porous metallic sheet without the use of a seeding layer. Forexample, a NaA zeolite membrane can be formed on a thin, porous metalsupport sheet without a seeding layer using a synthesis solution havinga molar composition of 10 Na₂O: 0.2 Al₂O₃:SiO₂: 200 H₂O. The solutionwas prepared by combining 0.2 g aluminum powder (200 mesh, 99.95+%,Aldrich) and 64.12 g DI water in a 250 ml polypropylene bottle andstirring for about 10 mins. 14.828 g of sodium hydroxide (Aldrich) wasadded and the bottle quickly capped. Stirring for additional 30 minutes.3.71 g Ludox LS30 colloidal silica (30 wt %, silica, Aldrich) was addeddrop-wise to the stirring solution. Solution was stirred forapproximately 4 hrs until it become clear.

The direct hydrothermal growth was performed in a reactor with the frontside of the porous Ni support sheet facing downward. The back side ofthe support sheet was covered with Teflon or another substrate to avoidcrystal growth. 18 ml of the NaA synthesis solution was poured into thereactor. The reactor was then sealed and heated in a pre-heated oven at65° C. for 7 hours. After the hydrothermal growth, the reactor wascooled down naturally in air. The NaA membrane sheet was taken out,rinsed under running D.I. water, dried with compressed air, and kept at50° C. overnight. The membrane was then heated at 400° C. for 4 hours in2 vol. % O2/N2 mixture. The rate for both heating and cooling were 1°C./min.

FIGS. 29A-29D show SEM micrographs obtained from a hydrophilic-typezeolite NaA membrane. FIGS. 29A and 29B show at two different scales thesurface of the membrane layer after hydrothermal growth. FIGS. 29C and29D are a cross-sectional views at two different scales of the membranesheet. A continuous zeolite film 301 having a thickness less than 2 μmis formed directly on the substrate 302 without a seeding layer. Thestructure of the membrane layer can be seen in the micrographs.Referring to the XRD spectrum in FIG. 29E, existence of the zeolitecrystal structure and composition is determined by the thin-film XRDmeasurement. This membrane was prepared by the direct growth techniqueas described above.

TABLE 9 Testing results of a water-selective membrane for water removalfrom air. Feed Side Pressure Permeance (mol/m²/s/Pa) H₂O/O₂ Separation(bar) H₂O O₂ Factor 1 6.7E−08 6.0E−10 108 2 2.9E−08 6.8E−10 43 3 1.5E−083.6E−10 43Testing conditions:

-   Feed side: 20 sccm of moisturized air (38.7% relative humidity, H2O    molar fraction 0.0051),-   Permeation side: atmospheric pressure, 40 sccm of He gas purge 24.5°    C.-   Separation temperature: 24.5° C.    Tables 9 and 10 show testing results for the water selective    membrane of FIG. 29 with respect to water removal from air and from    a water/ethanol mixture, respectively. Table 9 demonstrates that the    NaA membrane is selective toward water vapor. The H2O permeance is    about two orders of magnitude higher than the O2 permeance. However,    the water vapor permeance decreases with increasing feed side    pressure (or pressure gradient). This indicates that the membrane in    the feed side is saturated and water vapor permeation rate is    limited by diffusion rate of the adsorbed H2O rather than by    gas-phase diffusion of H₂O.

TABLE 10 Testing results of the water-selective membrane for removal ofwater from water/ethanol liquid feed. Perme- Sam- ation Water inH₂O/EtOH Separation Separation pling Flux Permeate separation Temp, ° C.Process Time, h (kg/m2/h) (wt %) factor 60 Pervapo- 24 0.046 89.6 78ration 90 Gas-phase 16 0.32 94.2 146 105 Gas-Phase 5.5 0.37 97.5 351Testing Conditions:Feed side: Continuous flow of 10 wt % water/ethanol liquid mixture,atmospheric pressurePermeate side: Vacuum (−12.86 psi)

The separation performance of the water-selective membrane was furthertested by feeding a water/ethanol liquid mixture into the membrane testcell. The feed mixture passes over the font side of the membrane. Thepermeate was pulled out by vacuum from the back side of the membrane andcollected in the liquid N₂ trap. The testing conditions and results arelisted in Table 10. At a separation temperature of 60° C., theseparation process can be viewed as pervaporation, that is, liquid phasein the feed side and vapor-phase in the permeate side. At 90 and 105°C., the liquid feed should be vaporized and thus, the separation occursin the gas-phase. The permeation flux significantly increases as thetemperature is raised from 60 to 90 and 105° C., while the H₂O/EtOHseparation factor increases at the same time. The results indicate thatthe present membrane performs well for gas-phase separation. The watercontent is concentrated to above 90% in the permeate side from 10 wt. %in the feed side.

TABLE 11 Testing results of the hydrophobic membrane for selectiveremoval of ethanol from ethanol/water liquid by pulling vacuum inpermeate side Separation Testing EtOH wt % EtOH flux, EtOH/H₂O Temp, °C. time, h in permeate kg/m²/h separation factor 60 14.5 28.6% 0.152 3.675 3.5 35.6% 0.256 5.0 90 3.5 59.9% 0.625 13.5Testing Conditions:Feed: 10 wt. % EtOH/water liquid flow, atmospheric pressurePermeate: vacuum (−12.7 psi)

TABLE 12 Testing results of the hydrophobic membrane for selectiveremoval of ethanol from ethanol/water liquid by gas sweep in permeateside Separation Testing EtOH wt % EtOH flux, EtOH/H₂O Temp, ° C. time, hin permeate kg/m²/h separation factor 60 2.5 62.3% 0.23 14.9 75 1.7555.7% 0.44 11.3 90 1.0 61.1% 0.84 14.1Testing Conditions:Feed: 10 wt. % EtOH/water liquid flow, atmospheric pressurePermeate: 100 sccm of He sweep gas flow, atmospheric pressure

The hydrocarbon-selective and/or alcohol-selective zeolite membraneshown in FIG. 28 was tested for ethanol/water separation by feeding 10wt % ethanol/water liquid into the membrane testing cell. Table 11 liststesting results by pulling vacuum in the permeate side, while results inTable 12 were obtained by use of a sweep gas on the permeate side. Thehydrocarbon-selective and/or alcohol-selective zeolite membrane isclearly selective toward EtOH permeation over H₂O. Both permeation fluxand EtOH/H₂O separation factor increases with separation temperature ifthe permeated is pulled by vacuum. By use of He sweep gas, thepermeation flux still increases with temperature but the separationfactor is nearly constant. The permeation flux with He sweep isconsistently higher than the vacuum pulling.

The impact of membrane preparation conditions on separation performanceof silicalite membrane sheets is illustrated by the experimental resultsin Table 13. Three membrane sheets were prepared with the same processsteps. Briefly, each porous Ni sheet was coated two times with seedcrystals of about 100 nm sizes dispersed in de-ionized water. The seedcrystals were precalcined at 600° C. for 5 hours in air prior to makingthe coating suspension. The seeded substrate was grown in an autoclavereactor with the same templated solution but under different conditions.Membranes 1, 2 and 3 were synthesized at hydrothermal reactiontemperatures of 140° C., 160° C., and 180° C., respectively. Aftergrowth, the membrane samples were treated 2 hours at 400° C. in 2% O₂/N₂flow with 1° C./min temperature ramp rate (profile 1). After calcinationat 400° C., membrane 3 was further heated in a pure hydrogen gas flow at600° C. for 2 hours with 1° C./min temperature ramp rate, that is,profile 2. The resulting membranes were tested on the same testingapparatus under the same conditions. 10 wt % EtOH in H₂O liquid was fedinto the membrane testing cell at flow rate of 1 cc/min underatmospheric pressure, while the permeate was removed by vacuum at 1torr. It can be seen that membrane 1, which was synthesized by 2 hoursof growth at 140° C., gave the highest permeation flux and goodselectivity.

TABLE 13 Impact of silicalite synthesis conditions on membraneseparation performance. Membrane # 1 2 3 Growth conditions 140 C., 2 h160 C., 2 h 180 C., 2 h Post treatment Profile 1 Profile 1 Profile 2Separation performance Temp, ° C. 75 75 75 Ethanol/water 22.8 30.0 2.0separation factor/ Total flux, kg/m²/h 1.69 0.91 0.68

The impact of the synthesis conditions on separation performance of theH₂O-selective zeolite membrane is shown by the experimental results inTable 14. The porous Ni substrate sheet was first coated with the NaAzeolite seed of about 1.0 μm crystal sizes and followed with the about0.3 μm zeolite crystal size. The zeolite membrane growth of the seededsubstrate was conducted in an autoclave reactor with a template-freegrowth solution. With the same seeded substrate and same synthesissolution, separation performance is dramatically affected by thehydrothermal growth temperature. Both H₂O flux and H₂O/ethanolselectivity were substantially increased by raising the growthtemperature from 90 to 100° C. However, further increasing the growthtemperature to 110° C. caused decline of H2O/ethanol selectivity.Several reaction processes occur simultaneously during the hydrothermalreaction process. Thus, the reaction conditions need to be wellcontrolled to obtain the optimum zeolite membrane structure.

TABLE 14 Impact of hydrothermal growth conditions on H2O-selectivemembrane performance Membrane # NaA 1 NaA 2 NaA 3 Growth conditions 90°C., 3.5 hrs 100° C., 3.5 h 110° C., 3.5 hrs Separation performance^(a)Flux, kg/(m² · h) 0.9 3.8 3.5 Ethanol in permeate, 50.16 0.33 12.84 g/LH₂O/EtOH 168 27,465 689 selectivity factor ^(a)The membrane separationperformance was characterized with 90 wt. % ethanol/water feed at 75° C.The feed was under atmospheric pressure, while the permeate side wasunder vacuum of ~1 torr.

Robustness and high quality of the H₂O-selective membrane prepared underpreferred conditions (NaA #2 in Table 14) have been demonstrated byseparation testing under various conditions. FIG. 30 shows that bothwater permeation flux and H₂O/ethanol selectivity increase withseparation temperature. In this set of testing, 10 wt % H₂O/ethanol feedwas introduced into the membrane cell under atmospheric pressure and thepermeate was removed by vacuum of 1 torr. The results clearly illustrateunique performance attribute of the zeolite membrane over a range ofseparation temperature. The membrane provided fairly high flux andselectivity at the testing temperature of 408K (135° C.). Mosttraditional polymeric membranes could not function at such hightemperatures. The trend of concomitant increase of flux and selectivityis exceptionally desirable for a practical separation application, sincemost membranes in prior arts were encountered with a trade-off betweenthe flux and selectivity, that is, decline of the selectivity withincreasing flux. FIG. 31 shows membrane separation performance withdifferent water content in the feed water/ethanol mixture at separationtemperature of 75° C. The H₂O permeation flux increases with the feedwt. %, which can be explained by the increased partial pressure gradientof water vapor across the membrane. The H₂O/ethanol selectivitydecreases with increasing wt % H₂O in the feed. This kind of performanceattribute is very desirable for deep drying by the membrane separation.However, the lowest selectivity is still around 6000 and is high enoughto meet practical separation application. Stability is critical forpractical application of any membranes. FIG. 32 shows variations of theflux and the water content of the permeate with time on stream. The H₂Oflux was stable around 4.0 kg/m2/h at separation temperature of 75° C.,rapidly increased to 7.5 kg/m2/h upon increase of the temperature to 90°C., and went back to the same flux level after the temperature wascooled down to 75° C. The results show rapid response of the membrane tothe change of separation temperature and stability of the membranestructure under these separation conditions. Compared to 90 wt % ethanolin the feed, the ethanol content in the permeate was very low, typicallybelow 0.05 wt %, which corresponds to a H₂O/ethanol selectivity factorabove 10,000.

TABLE 15 Comparison of H₂O-selective zeolite membranes (4A or NaA-type)for ethanol/water separation. Water Permeate in feed pressure Flux Ref.Support (wt %) T (K) (kPa) (Kg/m²/h) Selectivity # metal sheet 10 3630.1-0.2 7.5 >10,000 n/a metal sheet 10 348 0.1-0.2 4.0 >10,000 n/aalumina tube 10 348 0.7 5.6 >5,000 69 10 378 NA 4.5 >10,000 10 aluminatube 10 398  0.05 3.8 3,600 81 alumina tube 9.2 366 0.5 2.5 130 65 10348 NA 2.2 >10,000 10 5 348 NA 1.1 >10000 10 TiO₂ 10 323 0.2 0.8-1.08,500 13 alumina tube 10 353 NA 0.54 >10000 102 alumina tube 10 323 0.20.5 16,000 70 alumina tube 5 318 0.4 0.23 8,300 61 Silica tube 3 333 NA0.37 70 79 Ref. # in Table 5 of S.-L. Wee et al. Separation andPurification Technology 63 (2008) 500-516.

Referring to Table 15, the performance of various water-selectivezeolite membranes (4A or NaA-type) in water/ethanol separations aresummarized. The first two rows summarize the performance of 4A zeolitemembrane layers on porous metal support sheets according to embodimentsof the present invention. The performance data on the remaining membranematerials is summarized from that which was reported in Table 5 of S.-L.Wee et al. Separation and Purification Technology 63 (2008) 500-516. Forthe same-type zeolite material, ethanol/water separation performances ofthe membrane sheets formed according to embodiments of the presentinvention are significantly better than that which is reported in theprior art with respect to the combination of required flux andselectivity.

TABLE 16 Comparison of different membranes for ethanol/water separation.Water in Permeate feed pressure Flux Membrane Support (wt %) T (K) (kPa)(Kg/m²/h) Selectivity Ref. # NaA metal sheet 10 348 0.1-0.2 4.0 >10,000n/a NaA metal sheet 10 363 0.1-0.2 7.5 >10,000 n/a Mordenite aluminatube 10 423 0.5 0.2 139 a-35 Mordenite alumina tube 15 363 0.2 0.1 60a-68 NaX 10 348 NA 0.9 360 b NaY 10 348 NA 1.6 130 b Silica alumina disc10 353 0.6-0.8 1.0 800 a-8 Polymeric/Composites listed belowSilica/Acrylamide 10 323 NA 0.30 3200 b CMC (Na ion) 10 303 NA 0.05 2430b GFT 5 353 NA 0.01 9500 b PAA/polyion 5 333 NA 1.63 3500 b Chitosan 10333 NA 0.10 6000 b Polyimide 10 348 NA 0.01 850 b a. Ref. # in Table 5of S.-L. Wee et al. Sep. Purif. Technol. 263 (2008) 500-516. b. Y.Morigami et al. Sep. Purif. Technol. 25 (2001) 251-260.

Similarly, referring to Table 16, the membrane sheets of the presentexample exhibit better performance than many other membrane materialswith respect to the combination of required permeate pressure, flux, andselectivity. The first two rows of Table 16 summarize the performance of4A zeolite membrane layers on porous metal support sheets according toembodiments of the present example. No other materials exhibit the samelevel of high performance.

Zeolite membrane growth on the ceramics-modified porous Ni sheet werealso tested to demonstrate the limitations of using a ceramic transitionlayer. A layer of yittia-stabilized zirconia (YSZ) of mean pore sizesabout 200 nm could be deposited on the porous Ni sheet at thicknessabout 5 um by either screen printing or spray technique. A silicalitemembrane was grown on the porous zirconia surface by methods of thepresent invention as described elsewhere herein. However, the resultingmembrane film was readily peeled off from the Ni substrate. By contrast,the silicalite membrane directly grown on the bare Ni substrate asdescribed and pictured in FIG. 29 adhered to the substrate very well andcould not be peeled off. A complete set of data on the silicalitemembrane supported on porous metallic substrate sheet/plate/disk forethanol/water or hydrocarbon/water separation are scarce in the priorarts. Some complete sets of performance data for the H₂O-selectivemembrane supported on a metallic substrate are provided by Jafar et al.and by Holmes et al. Table 17 compares membrane characteristics andperformance data from embodiments of the present invention with those ofJafar and Holmes. The thickness, pore size, porosity, and pore structureof those two porous metallic substrate disks were not disclosed,although they are critical parameters for a zeolite membrane productconcept. The permeate pressure during separation tests of those twopapers was not disclosed either. With the same type of NaA zeolitematerial, the membrane prepared according to the present invention showsabout one order of magnitude higher water flux than those of Jafar etal., and Holmes et al. The H₂O/ethanol selectivity of the presentmembrane is a few orders of magnitude higher than those numbers reportedby Holmes et al. Jafar et al., tested water removal from awater/iso-propanol mixture. Iso-propanol is a larger molecule thanethanol. Fundamentally, H₂O/iso-propanol selectivity should be muchhigher than H₂O/ethanol selectivity for the NaA-type zeolite material,because NaA pore size is small enough to exclude iso-propanol fromadsorption into its pore. Table 17 shows that the present membraneprovides much higher H₂O/ethanol selectivity even than H₂O/iso-propanolobtained by Jafar et al. The comparison clearly shows criticalimportance of the features and properties resulting from embodiments ofthe present invention on membrane performance (flux, selectivity,adhesion) even for a same-type zeolite membrane material. The zeolitemembrane performance is much determined by the membrane sheet structureand on preparation methods.

TABLE 17 Comparison of the NaA membrane of this example to the same typeof zeolite membrane supported on porous metal disk in prior art. Waterin Permeate feed pressure Flux H₂O Membrane Support (wt %) T (K) (kPa)(Kg/m2/h) Selectivity Source NaA Thin porous 10 wt % 348 0.1-0.24.0 >10,000 present metal sheet H₂O in ethanol NaA same same 363 0.1-0.27.5 >10,000 present NaA porous 10 wt % 333 NA 1.3 to 0.7 1,800 to Jafar& zirconia/Ni/Cr H₂O in 8,000 Budd alloy mesh, iso- 1997 KA Cerameshsheet propanol 0.4 to 1.3 400 Jafar & (Acumen Ltd) to 1,500 Budd 1997NaA Stainless steel 10 wt % 298 NA 0.058 11.1 Holmes sinter (Alltech H₂Oin 313 NA 0.11 9.4 et al. Assoc.) ethanol 323 NA 0.135 9.4 2000 333 NA0.205 12.3 Jalal J. Jafar, Peter M. Budd “Separation of alcohol/watermixtures by pervaporation through zeolite A membranes” MicroporousMaterials 12 (1997) 305 311. S. M. HOLMES, M. SCHMITT, C. MARKERT, R. J.PLAISTED, J. O. FORREST, P. N. SHARRATT, A. A. GARFORTH1, C. S. CUNDYand J. DWYER “ZEOLITE A MEMBRANES FOR USE IN ALCOHOL/WATER SEPARATIONSPart I: Experimental Investigation” Trans IChemE, vol.78, Part A,pp1084-1088, 2000.

According to some embodiments, the individual membrane sheets disclosedin either Exemplary Embodiment 1 or Exemplary Embodiment 2 can beassembled into a mini-channel module. Referring to FIG. 33A, themembrane channels are formed by stacking two membrane sheets 701 frontside 702 to front side 702, wherein the zeolite membrane layers arefacing each other. Spacers 705 are placed between two membrane sheets toprovide mechanical support to the sheet and to also define a membranechannel 704. The spacers in the membrane channel are arranged in adirection perpendicular to those that may be in the permeate flowchannel 703. Permeate flow channels are formed by stacking the membranesheets back side to back side. Preferably, the back sides make baremetal to bare metal contact, however spacers can also be used. Themembrane sheets are stacked layer by layer though repeated process stepsto obtain the desired number of membrane channel layers. Finally, thewhole module is bonded together. Braze or sealer is applied on the twoends of the module to fill up any bypass voids from the membrane to thepermeation channel. For industrial production, the whole process can beautomated. Sealing glasses (e.g. alumino-barium-silicate based) andmetal base brazes (e.g. Ag-based) can be used or modified forpreparation of the mini-channel module.

The channel openings are a critical design parameter for the module.Selection of an optimum channel opening is a result of compromisebetween surface-area packing density, channel flow hydrodynamics, andpotential manufacturing cost. The small channel size has a high-surfacearea packing density and also reduces thickness of filtration boundarylayer. However, if the channel size is too small, flow hydrodynamicsinside the channel and manufacturing cost become a problem.Mini-channels that have a size greater than about 0.3 mm can typicallybe formed with low-cost material manufacturing processes while creationof mini-channels smaller than 0.3 mm can be fairly expensive at largescales.

The mini-channel modules are preferably hosted inside a pressureenclosure. FIG. 33B shows a schematic of an exemplary membrane package710. The feed stream 712 is divided into a water stream 711 and aconcentrated stream 713 through the filtration vessel. A positivepressure gradient (1˜25 bar) between the feed stream and the permeatestream is exerted during separation tests. A metallic vessel made of thesame material as the membrane support is preferred. The membrane packageis critical to durability and separation performance, particularly forhigh-pressure operation. Both flow and stress distribution is affectedby the package method. The flow distribution is directly related toeffective utilization of all membrane channels. The stress distributiondetermines mechanical integrity and durability of the testing cell.

The membrane sheets may also be packaged into a simple plate-typemembrane module as illustrated in FIG. 34. Two membrane sheets 801 aresealed back side to back side, with or without a spacer, into a platehaving an internal opening 803. The zeolite membrane-coated surface 802is exposed to the feed water/hydrocarbon mixtures 804. The targetedmolecule is removed from the mixture as permeate 805 though the membraneinto the inner open space of the membrane plate and pulled out of themembrane module by vacuum and/or gas purge. For a given membrane sheetthickness, the width of the internal opening is the major designparameter. To pack more membrane area in unit volume, a thinner plate ispreferred. To minimize the flow resistance to withdraw the permeate outof the membrane module, a thicker plate is preferred. Preferably, thewidth of the internal opening is between 0.3 to 6 mm.

V. Exemplary Embodiment 3—Thin-Sheet Faujasite-Type Zeolite Membranesand Preparation Methods A. Introduction

Faujasite frameworks containing 12-member rings (each member=one Si orAl atom) are one class of zeolite materials with significant industrialuses. X and Y are two types of the Faujasite zeolite materials commonlyused as catalysts and adsorbents. They provide a larger channel opening(0.7-0.8 nm) than the NaA-type zeolite (0.4 nm). Type-X Faujasite has anormal composition of Na₈₆Al₈₆Si₁₀₆O₃₈₄:wH₂O (w˜₂₆₀), while type-YFaujasite has a normal composition of Na₅₆[Al₅₆Si1₃₆O₃₈₄]:250 H₂O. Na⁺¹in the Faujasite-zeolite can be exchanged with other metal ions totailor the channel opening and surface chemistry for specificapplications. For example, NaX is a common adsorbent for CO₂ and H₂Ocapture from various process gas streams and LiX is an adsorbent for airseparation. The HY zeolite is used as a catalyst for fluid catalyticcracking processes. The faujasite has good thermal and chemicalstability. The framework structure is typically intact in presence ofvarious impure gases (SO₂, H₂S, COS, NO, H₂O, CH₄, oil, etc.).

Although methods of preparing Faujasite-type zeolite membranes have beenpreviously disclosed, none provide specifications for optimum seedingcrystals and seed coating methods for scale-up of membrane preparationprocesses. The growth temperatures were typically around 90-100° C. andthick porous alumina disks or tubes were often used in the preparation.The seed crystals and seeding methods varied dramatically amongdifferent research groups.

B. Description of Present Membrane Structures and Preparation Methods

The Faujasite membrane structure and preparation method are similar towhat were described above for Exemplary Embodiment 1. The maindifferences lie in using of Faujasite-type zeolite as the seedingcrystals and Faujasite-type growth solution for secondary growth. Asoutlined in FIG. 35, a porous, robust flat sheet support such as porousmetal sheets of thickness less than 200 μm with suitable surfacetextures is chosen as a support. The robustness means that the supportsheet maintains its mechanical integrity during handling, seed coating,and hydrothermal growth. The suitable surface textures mean that thesurface of the support sheet to be deposited with the zeolite membraneis free of any major defects, such as holes and cracks above 5 μm, withsurface porosity from 50 to 15%, preferably 45 to 20%.

A seed coating solution is prepared by suspending the parent Faujasitezeolite crystals in a water-based solution in such a way that thesuspension is homogenous and would not result in sedimentation orsegregation during the seed coating process. Thus, average particlesizes of the seeding crystals are preferred from 0.5 to 2.0 μm and solidloading in the suspension is preferred to be 0.5 to 5.0 wt %. The seedcoating solution can be prepared by ball-milling or attrition milling ofparent Faujasite powder in the water-based solution. Organic solventssuch as alcohols and dispersion agents such as PEG and PVP may be addedinto the solution to stabilize the suspension and/or modify theliquid/support contact angle. Pure water or liquid containing >50 wt. %water is preferred as the carrier or dispersing fluid for preparation ofthe seed coating solution.

The support is coated with the seed solution by use of some simple,scalable methods, such as spray coating and dip coating. The coatingconditions can be controlled in such a way that the seed crystals areuniformly dispersed on the support at a loading from 0.1 to 0.5 mg percm² of the support surface and some seeding crystals are allowed topenetrate into the support pore. The support surface is exposed to thecoating solution for a certain time in a range of 0.1 second to 1minute. Full coverage of the support surface by the seed solution ispreferred. However, presence of excessive solution on the supportsurface should be avoided. The coating may be performed several timeswith intermittent drying to obtain uniform coverage of the support.

The seeded support sheet is loaded in a growth reactor after drying andimmersed in the suitable growth solution to grow a continuous, denselayer of the zeolite membrane out of the seeded support. A number of theseeded sheets can be loaded in one growth reactor. The growth reactor isdesigned and built in such a way that uniform temperature profiles overall the membrane sheets are maintained during the growth process. Thegrowth solution is water-based with compositions tailored for growth ofthe Faujasite-type zeolite crystals. The growth solution preparation isdescribed as follows. An Al sol is prepared from Al precursors such asAl(OH)₃ with suitable Al and Na content. A Si sol is prepared from theSi precursor such as sodium silicate with suitable Si and Na content.Then, the Al sol is mixed with the Si sol, preferably at roomtemperature to obtain a homogeneous solution without significantsegregation or precipitation within time period of growth, typically 1to 4 hours.

The hydrothermal growth is conducted under suitable conditions so thatall the inter-particle (or crystals) voids are closed by secondarygrowth of the seeding crystals without excessive deposit of the zeolitepowder or crystals on the surface. The membrane growth weight gain ispreferred to be 0.5 to 3 mg/cm² of support area. The preferred membranegrowth conditions are 90 to 110° C. for 1 to 7 hours. If the growthtemperature is too high and/or growth time is too long, excessiveamounts of zeolite may deposit on the support surface, which can causecracks and/or reduce the membrane permeance. If the growth temperatureis too low and/or the growth time is short, inter-crystal voids orspaces would not be closed that the membrane would not have highselectivity. After growth, the membrane sheet is rinsed with water toremove any growth solution and solid deposit on the surface, and therinsed membrane is left to dry prior to usage.

C. Examples

i. Faujasite Membrane Preparation by Use of Commercial Seeding Crystalsand Growth Solutions Previously Known

This group of studies was conducted using NaX and NaY powder acquiredfrom commercial sources as the seeding crystal, and with membrane growthsolutions being prepared by mimicking the procedures reported in theliterature. FIGS. 36A and 36B show that the as-received powder compriseslarge agglomerates of the crystals. As-received NaY powder (HS-320,Wako) was milled to average particle size of 3.85 μm. To avoiddegradation of the parent zeolite crystals due to excessive milling,moderate ball-milling was conducted using a slow rolling speed. 6.0 g ofthe NaY powder was mixed with 150 g of deionized water and added with 50milling beads. The mixture was milled overnight at 30% set point of theball-milling speed scale. Then, additional water was added to obtain 200cc of a seed coating solution with 3 wt % solid loading. The as-receivedNaX (Wako, F-9) was milled in the same way to average particle size of2.5 μm. The atomic compositions of the as-received zeolite powder arecompared to the milled one in Table 18. As-received NaY has a higherSi/Al ratio than as-received NaX. Si/Al ratio of the milled NaYparticles sampled from the seeding solution was approximately same asthat of the as-received one. However, the milled NaY had a lower Al/Naratio than the as-received one, indicating possible Na loss from theframework.

The 13 cm×13 cm porous Ni support sheets of about 40% porosity werecoated with four different seed coating solutions to assess impacts ofthe seeding crystals on the membrane growth. The results are summarizedin Table 19. Two seeding solutions were prepared from each parentzeolite powder through ball-milling and attrition-million, respectively.The seed coating was conducted by following the spray coating procedureused previously for NaA membrane preparation. The support sheet wascoated two times with each seeding solution with intermittent drying toobtain the seed loading at about 0.2 mg/cm². All the seeded sheetslooked fairly uniform.

TABLE 18 SEM/EDS elemental analysis of as-received and milled zeolitecrystals as seeds NaX Element (F-9) NaY NaY seed solution prepared byball-milling atomic As As- Stable suspension % received received Spot 1Spot 2 O 66.16 65.07 63.79 63.3 Na 7.59 9.81 5.87 8.02 Al 12.46 8.079.24 8.87 Si 13.79 17.05 21.09 19.81 Si/Al 1.11 2.11 2.28 2.23 ratioNa/Al 0.61 1.22 0.64 0.90 ratio

TABLE 19 Seed coating with milled commercial NaA and NaY powder Spray1st total Poros- Seeding coating time seeding, seeding, Support ID ity %solution (s) 1^(st), 2nd mg/cm² mg/cm² 61443-60-13 42.9 ball-milled 175,165 0.11 0.21 61443-69-14 39.3 NaX 0.11 0.21 61443-70-7 39.3 ball-milled 175, 190 0.08 0.17 61443-71-7 41.4 NaY 0.08 0.16 61443-71-1441.9 attrition- 175, 140 0.12 0.23 61443-71-16 40.8 milled NaX 0.12 0.2261443-71-10 44.4 attrition- 175, 130 0.13 0.21 61443-71-12 42.7 milledNaY 0.11 0.19

Several growth solutions were prepared to assess impact of the growthsolution composition and preparation on membrane growth. Table 20 liststhe solution number and corresponding molar composition. Raw materialsused for the solution preparation are NaOH, de-ionized water, sodiumaluminate (anhydrous, Sigma), silica sol (Ludox LS colloidal silica, 30wt %, Sigma-Aldrich), aluminum sulfate hydrate (Al₂(SO₄)₃×H₂O, 98%,Sigma-Aldrich), sodium metasilicate nona-hydrate (Na₂SiO₃ 9H₂O, 98%,Sigma-Aldrich), sodium silicate solution (˜10.6% Na₂O, ˜26.5% SiO₂, 1.39g/cc, reagent grade, Sigma-Aldrich). 4.7 wt % Al(OH)₃ solution waspre-made according to the recipe previously used for preparation of NaAmembrane. 34.09 g of Al(OH)₃, 61.488 g of NaOH, and 630 g of H2O weremixed at 95° C. for 2 hours to obtain a clear, homogeneous solution.Then, the solution was cooled to room temperature for growth solutionpreparation.

TABLE 20 Molar ratios of the growth solutions prepared in this exampleSolution # SiO₂ Al₂O₃ Na₂O H₂O Note 2.1 1.0 0.11 9 555 JMS 354 (2010)171-177 2.2 1.0 0.10 1.4 84 Microporous and Mesoporous Materials 130(2010) 38-48 2.3 1.0 0.10 1.4 84 Same composition and Si sol as solution2.2. A different Al sol used 2.4 1.0 0.11 9 555 Same composition assolution 2.1. Different Al and Al sols used 2 1.0 0.20 2.8 140 Same Aland Si precursors 4 1.0 0.075 3.73 202 used as solution 2.4 but 5 1.00.10 1.4 70 with different concentrations

Growth solution 2.1 with molar ratio of Si:Al:Na:H₂O=1:0.22:18:555 wasprepared by following the procedure reported in (JMS 354 (2010)171-177). 13 g of NaOH was dissolved in 110 g of de-ionized water, towhich 0.50 g of sodium aluminate was added and mixed to obtain a clearAl sol at room temperature. To prepare the Si sol, 5 g of NaOH wasdissolved in 130 g of water, to which 5.0 of the Ludox LS silica sol (30wt. %, Bayer AG) was added. The mixture in a glass beaker was heated to40° C. for 30 min and then to 80° C. for about 10 min under stirring.The beaker was removed from the heater after a clear solution wasformed. After the Si sol was cooled to room temperature, the Al sol wasadded drop-wise to the Si sol under stirring.

Growth solution 2.2 with molar ratio of Si:Al:Na:H₂O=1:0.20:2.8:84 wasprepared by following the procedure disclosed in Microporous andMesoporous Materials 130 (2010) 38-48 by Mosca et al. This is a muchhigher concentrated solution, compared to solution 2.1. Six grams ofaluminum sulphate was dissolved into 100 g of water and mixed to obtaina clear solution, to which 5.5 g of NaOH was added to obtain the Al sol.Five grams of NaOH was dissolved in 130 g of deionized water, and addedwith 48.0 g of sodium metasilicate hydrate to make the Si solution. Theresulting mixture was cloudy and was heated to 80° C. to get a clearsolution. After the Si solution was cooled down to room temperature,74.11 grams out of 110.46 grams of the Al solution was added drop-wiseto obtain a homogeneous growth solution.

Growth solution 2.3 with the same molar ratio as solution 2.2(Si:Al:Na:H₂O=1:0.20:2.8:84) was prepared with the same Si solution asused in solution 2.2, but different Al solution. The Al solutionpreparation procedure was as follows: 10.0 g of NaOH was dissolved 50 gof water, to which a 70 g of the 4.7 wt % clear Al(OH)₃ solution wasadded and mixed to obtain a uniform clear solution. The Al solution wasadded into the Si sol drop-wise during stirring.

Growth solution 2.4 with the same molar ratio as solution 2.1 wasprepared with different Si and Al solutions from what were used forsolution 2.1. One hundred and seventy grams of NaOH was dissolved into110.0 grams of water, added with 13.0 g of the 4.7 wt % clear Al(OH)₃solution, and mixed to obtain a clear Al solution. Eight grams of NaOHwas dissolved 12.0 grams of water, added with 8.5 grams of the 26.4 wt %sodium silicate solution, and mixed to obtain a clear Si solution. TheAl solution was added into the Si solution drop-wise during stirring atroom temperature.

Growth solutions 2, 4, and 5 were prepared by using pre-made 4.7 wt %Al(OH)₃ solution and 26.4 wt % sodium silicate solution as respective Aland Si precursor. For each growth solution preparation, the pre-madeAl(OH)₃ solution and sodium silicate solution were added with designedamounts of NaOH and/or de-ionized water to obtain the Al and Sisolution, respectively. The H₂O/Na ratios in the Al and Si solutionswere maintained approximately same. In a standard mixing procedure, theAl solution was added into the Si solution drop-by-drop under stirringat room temperature.

The membrane growth was conducted with the above seeded sheets andabove-prepared growth solutions under different conditions. The resultsare summarized in Table 21. The growth was conducted in the planarreactor in the same way as used for the NaA membrane growth of presentdisclosure. In the planar reactor, the reactor wall was directly heatedby electrical heaters and a uniform temperature profile can be obtained.To simulate the Faujasite membrane growth used in the literature,membrane growth was also conducted in a glass autoclave reactor that washosted inside an oven. For the planar reactor growth, the growthtemperature was ramped from room temperature to a designated temperatureat 1° C./min. For the oven growth, the reactor was heated by the oventhrough radiation heat transfer and the ramping temperature rate was notcontrolled. After growth, the membrane sample unloaded from the reactorwas rinsed with tap water first and then with de-ionized water. Thespent growth solution was collected and observed for comparison to thefresh one. The dried membrane was checked for leakage by use of Red40food color. The colored water was dropped on the membrane surface toobserve any color penetration on the other side of the membrane sheet.If the membrane has major defects, the color will leak through.

TABLE 21 Summary of membranes grown on porous Ni sheets seeded withcommercial parent zeolite crystals Support Spent Growth Mem ID IDSeeding Growth solution Growth solution gain, Leakage Crystal 61509-61443- crystal # State conditions state mg/cm² check phase 159-5 60-13NaX ball 2.4 Clear 100° C. 4 h clear 1.62 Leak milled in planar 159-771-14 NaX 2.4 Some reactor clear 1.14 Leak on other attrition cloudy topphases milled corner 161- 71-14 NaX 2.1 clear clear 1.19 very 159-3attrition little milled leak 164- 71-16 NaX 2.2 Some few 3.99 No leak159-3 attrition cloudy solids milled 166- 71-16 NaX 2.4 Some 110° C. 4 hclear 1.33 No leak other 159-3 attrition cloudy in planar phases milledreactor 167- 71-16 NaX 2.2 Some few 1.11 No leak other 159-3 attritioncloudy solids phases milled 166- 71-16 NaX 5 clear 140° C. 4 h clear2.16 No leak other 159-6 attrition in oven- phases milled hosted reactor159-2-1 61443- NaX 2 Clear 120° C. 4 h clear 0.70 No leak other 71-16attrition in oven- phases milled hosted 159-2-2 71-7 NaY ball 2 Clearreactor clear 0.18 Some milled leak 159-2-3 71-16 NaX 4 Clear some 1.69No leak Some attrition precipitate Faujasite milled 159-2-4 71-7 NaYball 4 Clear some 1.53 Leak on Some milled precipitate some Faujasitespots

Except for solutions 2.2 and 2.4, all the other fresh growth solutionslooked clear. The solutions 2.2 and 2.4 looked mostly clear, but withsome cloudy phase. After growth, some spent solutions remained clear,while some spent solutions showed presence of solids and precipitates.Significant growth weight gain was obtained with the NaX-seeded supportsheets. With the same growth solution and under the same growthconditions, the attrition-milled NaX seeding crystals tend to yield moregrowth weight gain than the ball-milled one. Little or no growth wasfound with most NaY-seeded support sheets as evidenced by the weightgain. Those membrane samples showed rapid leakage of red 40 colormolecules. Only the membranes grown on the NaY-seeded support withgrowth solutions 2 and 4 at 120° C.-oven temperature showed someblockage to color penetration.

A few of membrane samples were selected to identify the crystal phasesby XRD analysis. For comparison, the parent seeding crystals, i.e.,as-received NaX and NaY powder, were analyzed first. FIGS. 37 and 38show XRD patterns for NaY and NaX powder, respectively. The dominatingcrystal phase identified for the NaY powder is Na₅₆ (Al₅₆Si₁₃₆O₃₈₄)—NaYcrystal phase. Aluminum silicate is another crystal phase possiblyexisting in this powder. Presence of minor SiO₂ crystal phase is found.For NaX powder, Na₅₆ (Al₅₆Si₁₃₆O₃₈₄)—NaX crystal phase is identified asthe dominating crystal phase and sodium aluminum silicate(Na_(1.84)Al₂Si_(2.88)O_(9.68)) is another crystal phase present in thissample.

However, no or little amounts of faujasite crystal phases were foundpresent in the resulting membrane samples. Only the last two membranesamples in Table 21 showed presence of some Faujasite crystal phases,while all the other membrane samples measured showed crystal phasesother than Faujasite. As a conclusion, Faujasite membranes could not beobtained by use of the commercial parent zeolite powder as the seedingcrystals and growth solutions reported in the literature. The Faujasitemembrane could not be obtained even by changing the growth conditionsand modifying the growth solution preparation.

ii. Preparation of NaX Membranes with In-House Seeding Crystals withNewly Developed Growth Solution

A growth solution preparation procedure was developed to prepare theseeding crystals and conduct secondary growth. The raw materials werede-ionized water, sodium hydroxide (99+% NaOH), alumina trihydrate(Merck, 65% Al₂O₃), sodium silicate solution (27.35% SiO₂, 8.30% Na₂O,1.37 g/mL). The preparation procedure is described as follows. Firstly,100 grams water plus 100 grams sodium hydroxide was mixed to obtain aclear solution denoted as solution 1. Secondly, solution 1 was addedwith 97.5 grams of alumina trihydrate powder and stirred at 100° C.until complete dissolution. The solution was cooled down to 25° C. toobtain a clear solution, which is denoted as solution 2. Thirdly,solution 3 was prepared by mixing solution 2 with 202.5 grams water.Fourthly, 100 grams of solution 3 was mixed with 612 grams water and59.12 grams sodium hydroxide to complete dissolution of the solid. Theresulting solution turned to slightly cloudy and is denoted as solution4. Fifthly, 219.7 grams of sodium silicate solution was mixed with 612grams water and 59.12 grams sodium hydroxide until total dissolution ofthe solid. The resulting solution was denoted as solution 5. Finally,solution 4 was poured into solution 5 under rapid stirring (˜700 RPM) toobtain a homogenous, milky mixture, which is denoted as solution 6. Ithad a molar composition of Si:Al:Na:H₂O=1:0.26:4.36:82.

The feasibility to grow Faujasite crystal phases with the solution 6prepared above was verified by heating the solution in a glassauto-clave reactor inside an oven at 90° C. for different times. Aftergrowth, the solid was separated from the spent solution bycentrifugation at 3500 rpm. The solid precipitate was rinsed withde-ionized water and dried. The dried samples were analyzed by SEM andXRD. SEM pictures in FIGS. 39A-39C show crystal morphologies of the NaXpowder obtained with three different holding times at 90° C. The atomiccompositions of the powder samples were assessed by EDS analysis. FIG.40 shows the XRD patterns of the three samples in comparison to the NaXpowder acquired from the commercial source. 4-hold resulted in powder ofsmall, uniform particles but without any XRD peak. Thus, 4-h hold at 90°C. was not in-sufficient to form zeolite crystals. Clear XRD peaksemerged when the holding time was increased to 8 and 20 hours. The XRDpatterns for the two samples grown with 8-hours and 20-hours holdingtimes completely matched each other, which indicates the same crystalphase formed at different holding times. Zeolite crystals are clearlyseen in the SEM pictures. The zeolite crystals tend to formagglomerates. The agglomerates became larger as the holding time wasincreased from 8 to 20 hours. The XRD peaks of the NaX powder grown inthis example match those of the commercial NaA sample. Compared to FIG.36B, FIGS. 39B and 39C show that the NaX crystal sizes grown in thisexample are substantially smaller than the commercial one. Crystal phaseanalysis shows that Na_(57.7)Al_(57.7)Si_(134.3)O₃₈₄ is the dominatingcrystal phase identified from the samples grown with 8-h and 20-hholding times. Presence of the other crystal phases was too low to bedetermined.

Four seed coating solutions were prepared with NaX powder prepared inthis example, which are named as 4-h wet, 8-h wet, 20-h wet and 8-h dry.The first three solutions were prepared from the wet solids grown with 4h, 8 h and 20 h holding times, respectively. The solid cake as separatedfrom the growth solution was mixed with a proper amount of de-ionizedwater to 3 wt % solid loading on dry basis. The solid loading on drybasis was determined by drying a small portion of the wet cake in avacuum oven at 100° C. for 24 hrs. The last solution was prepared byball-milling of the 8-h wet solid dried in the vacuum oven at 100° C.for 24 hours. 5.95 grams of the dried powder was mixed with 80 g ofwater and 27.1 cm³ zirconia beads in a 125 ml polypropylene bottle andball-milled at 50% speed of the full scale for 19.5 hours. Aftermilling, 130 g of de-ionized water was added to obtain 3 wt % solidloading.

13 cm×13 cm porous Ni sheets were coated with the above seed coatingsolutions using the established spray coating procedures. Three times ofspray coating were conducted on each support sheet to obtain uniformcoverage and targeted seed loading level. The results are summarized inTable 22. Each seeded sheet was cut into four portions for membranegrowth with different holding times. The growth solution was preparedusing the same procedure as described above for seeding crystal growth.The growth solution has atomic ratio of Si:Al:Na:H₂O=1:0.26:4.36:82.33.The four sheets of different seed crystals were loaded in each reactorrun. The three growth batches were conducted with holding times at 90°C. of 0, 3.5, and 16 hours, respectively. The temperature ramping ratewas at 1° C./min.

All the four sheets showed significant weight gain and presence of amembrane layer even with 0-hour holding at 90° C. It is interesting tonote that for the same seeded sheet and with the same growth solution,the growth weight gain tend to decrease with increasing holding time.This can be explained by deposition and/or adsorption of no-crystallinesold on the support sheet at 0-h holding time. Only the membrane grownwith 8-hours dry seeding showed no color leakage at the three differentholding times, while all the other membranes except for membrane#56-55-4 leaked the color. Membrane #56-55-4 was grown with 0-hoursholding over the 4-hours wet seeded support. It's very high grown weightgain can be attributed to deposit of amorphous solid materials ratherthan to the zeolite membrane growth. For comparison purposes, theNaA-seeded sheet was grown in the 3.5-hours batch together with theFaujasite membrane samples. Resulting NaA membrane showed the colorleakage. It indicates the need to use the NaA-type growth solution toobtain a NaA membrane.

TABLE 22 Faujasite membranes grown on porous metal support sheets seededwith seeding crystals prepared in this example Seed loading Growth from1^(st) weight Mem Spray and 2^(nd) Final seed gain, Note of ID SupportSeeding coating coating, loading, Growth mg/cm² Leak membrane 60037-porosity % crystals time (s) mg/cm² mg/cm² time, h change test surface56-55-1 43  8-h wet 100, 0.16 0.22 0 1.29 Leaked dark and 100, 90uniform 56- 44  8-h dried 100, 0.16 0.24 0 1.43 No white 55-2 100, 91leak powder 56- 47 20-h wet 100, 0.16 0.22 0 1.30 Leaked some white 55-3100, 75 powder 56- 45  4-h wet 100, 0.13 0.21 0 3.32 No lots of 55-4100, leak white 100 powder 57- 43  8-h wet 100, 0.16 0.22 3.5 1.29Leaked dark and 55-1 100, 90 uniform 57- 44  8-h dry 100, 0.16 0.24 3.51.23 No some white 55-2 100, 91 leak powder 57- 47 20-h wet 100, 0.160.22 3.5 1.07 Leaked dark, little 55-3 100, 75 white 57-55-4 45  4-h wet100, 0.13 0.21 3.5 1.20 Leaked dark, little 100, white 100 57- 43 NaA 90, 115 0.16 0.16 3.5 1.42 Leaked some white 55-6 powder 56-55-6 43 8-h wet 100, 0.16 0.22 16 0.81 Leaked white gray 100, 90 surface 56- 44 8-h dried 100, 0.16 0.24 16 1.80 No white 55-7 100, 91 leak surface 56-47 20 h wet 100, 0.16 0.22 16 1.08 Leaked lots of 55-8 100, 75 whitepowder 56- 45  4-h wet 100, 0.13 0.21 16 0.88 Leaked some white 55-9100, powder 100

Surface textures of the seeded sheet before and after growth arecompared in FIGS. 41A-41H. The pictures on the left side show thesupport surfaces seeded with 4-hour wet, 8-wet, 8-hours dry, and20-hours wet seed coating solutions, while the pictures on the rightside are the membrane surface after 3.5-hours holding time. The presenceof distinctive seeding crystals is clearly present on the 8-wet, 8-hourdry, and 20-hour wet seed-coated surface. Crystal inter-growth can beseen on all the four membranes after growth. However, only the membranegrown on the 8-hour dry seed-coated surface shows a dense texture. Themembranes grown on 8-hour wet and 20-hour seed-coated sheets showpresence of some un-covered pores. In the membrane grown on the 4-hourwet seed-coated surface, there are some spots comprising no-crystallinephases.

Impacts on the growth time on membrane formation on the 8-hour dryseed-coated surface are shown in FIGS. 42A-42D. It looks that a densemembrane layer could be formed with 0-hour hold. The membrane grown with3.5-hours hold has the least amount of deposition of largeparticles/crystals.

Atomic compositions of the powder materials used to make seed coatingsolutions, seed-coated sheets, and membranes are measured by EDS andlisted in Table 23. The corresponding areas/spots for EDS analysis canbe found in FIGS. 41A-41H and 42A-42D. The 8-hour and 20-hour grownpowder showed similar compositions, which are very different from the4-h grown powder. All the four seeded support sheets show significantpresence of Ni, which supports that the Ni support sheet was not fullycovered. After growth, Ni at % on the membrane-covered area/sport ismuch less than the corresponding seeded sheets. However, Ni at % levelis very high on the spot that is not covered by the membrane coating.The EDS analysis is consistent with the surface textures.

XRD analysis was performed on the selected membrane samples. FIGS. 43and 44 shows the XRD patterns of the membranes grown on the supportseeded with wet zeolite powder. The large peaks at 2θ.43° are the Nimetal phase coming from the support. The membrane peaks well match withthese of the powder material—parent seeding crystal. XRD patterns of themembranes grown on the support coated with the 8-h dry seed solutionwith different times also match the parent seeding powder well. The8-hour dry seed and 8-hour wet seed do not make any significantdifference in the membrane crystal phase. It is worth to note thatalthough the 4-hour wet seed did not have any crystal phase, themembrane has clear crystal phases. The phase identification shows thatthe dominating crystal phases for all the membrane samples are Y-typeNa₅₆[Al₅₆Si₁₃₆O₃₈₄] (Si/Al=2.43): 250H₂O with lattice parameter 24.75and X-type Na_(2.06) Al₂Si_(3.8)O_(11.6) (Si/Al=1.9): 8H₂O with latticeparameter of 24.77, i.e., Faujasite-type crystal phases. Thus, formationof Faujasite zeolite membranes with the seeding and growth methodstaught by the disclosed methods is demonstrated.

It is believed that the membrane textures can be optimized by adjustingthe preparation conditions to meet specific application needs.

TABLE 23 SEM/EDS analysis of seeding powder, seeded sheets, andmembranes Sample Atomic % Atomic ratio Note Note O Na Al Si Ni Na/AlSi/Al Powder material grown with different growth times 4-h powder 51.47.8 16.6 24.2 0.47 1.46 8-h powder 62.3 13.7 11.5 12.6 1.19 1.10 20-hpowder 61.8 10.3 13 14.9 0.79 1.15 Seed-coated Ni sheet 4-h wet coating34.0 14.8 6.0 7.7 37.6 2.44 1.27 8-h wet coating 31.2 12.1 10.5 11.934.2 1.15 1.13 8-h dried coating 36.5 12.4 10.7 11.9 28.4 1.16 1.11 20-hwet coating 28.7 11.1 8.2 8.9 43.1 1.36 1.08 Membrane samples 3.5-h holdArea 56.8 9.8 13.2 19.2 1.0 0.75 1.46 on 4-h wet seed 3.5-h hold whole50.1 11.8 12.9 17.0 8.0 0.92 1.32 on 8-h wet spot 1 54.2 9.3 13.9 20.61.9 0.67 1.48 seed spot 2 8.7 2.6 1.4 1.7 85.5 1.86 1.22 0-h hold 50.811.1 14.0 19.5 4.7 0.79 1.39 3.5-h hold 8 h dry 52.5 10.9 13.2 19.1 4.20.83 1.44 16-h hold seed 53.5 10.1 13.4 19.2 3.8 0.75 1.44 3.5-h holdspot 1 49.9 9.5 14.6 19.6 6.5 0.65 1.34 on 20-h spot 2 6.4 1.7 1.2 1.489.2 1.35 1.15 wet seed

iii. Molecular Sieving Performances of Faujasite Zeolite Membranes

The molecular separation functions of the Faujasite zeolite membranesprepared in Example ii were characterized with air dehumidification andCO₂ gas separation tests. The air dehumidification tests were performedin the same way as used previously for testing of NaA-type membranes.Humid air was introduced in the feed side of a membrane test cell underatmospheric pressure, while the permeate side was pulled vacuum. Thetesting results are summarized in Table 24. All the membranes showedsome selectivity toward H₂O permeation over air, which confirmsH₂O-philic nature of this-type membrane. The H₂O permeance for a fewmembrane samples with low H₂O/N₂ separation factor was too large to bemeasured on the present testing system. The two membrane samples,−56-55-2 and −57-55-2, showed excellent H₂O/air selectivity (separationfactor greater than 200) and very high H₂O permeance (>2.0E-5mol/m²/s/Pa) with feed air containing 3.33 mol % H₂O (˜85% relativehumidity). The H₂O/N₂ selectivity deceased with decreasing humidity inthe air. Thus, the air dehumidification mechanism appears similar to theNaA-type membrane. Selective H₂O adsorption into the zeolite pore blockspermeation of air. These two membranes were grown on the support coatedwith the 8-hour dry seed with 0.0 and 3.5-hour holding times. Themembrane grown on the identical seeded sheet but with 16-hour growthholding time, −56-55-7, gave H₂O/N₂ separation factor of only 13,drastically less than the other two membranes. The results indicate thatover-grown of the membrane may cause some defects in the membrane.Several membrane samples showed N₂/O₂ separation selectivity.

The three membranes grown on the 8-hour dry seeded support were furthertested with a CO₂ gas mixture. The results are listed in Table 25. Allthe three membranes showed CO₂/N₂ selectivity. The membrane grown with3.5-hour hold showed higher CO₂/N₂ selectivity than the other twomembranes, which suggests that separation performances for a specificapplication can be optimized by adjusting preparation conditions. When3.33 vol % H₂O was introduced into the CO₂ feed gas, CO₂, N₂, and O₂permeance dramatically decreased as H₂O permeance dominated. Thegas-separation test results demonstrate that Faujasite membranes can beused to separate individual molecules or for gas separation.

TABLE 24 Air dehumidification performances of Faujasite zeolitemembranes prepared in Example ii. Permeate Separation Mem# x_(H2O) inTemp pressure Permeance, mol/m²/s/Pa factor 60037- feed gas (° C.)(mbar) H₂O O₂ N₂ H₂O/N₂ O₂/N₂ 56-55-1 0.0333 32.9 36 Too large 7.2E−067.1E−06 2.7 1.01 -56-55-2 0.0333 32.3 5 2.6E−05 2.7E−09 4.4E−09 23610.63 0.0163 34.8 5 1.3E−05 1.6E−08 2.3E−08 299 0.69 0.0082 32.3 51.2E−05 9.2E−08 1.4E−07 44 0.68 -57-55-2 0.0333 29.8 5 2.3E−05 1.4E−082.8E−08 316 0.48 -56-55-7 0.0333 39.2 5 1.9E−05 5.6E−07 6.8E−07 13.40.83 -57-55-1 0.0333 30.9 5 3.7E−05 2.2E−06 2.3E−06 5 0.96 -56-55-60.0333 31.5 25 3.2E−05 2.4E−06 2.5E−06 4 0.95 -56-55-3 0.0333 30.9 46Too large 3.7E−06 3.7E−06 5 1.01 -57-55-3 0.0333 30.3 48 Too large3.8E−06 3.7E−06 5 1.03 -57-55-4 0.0333 30.4 25 4.3E−05 2.3E−06 2.4E−06 50.95

TABLE 25 Separation performance of Faujasite-zeolite membranes preparedin example ii for CO₂ gas mixture (15% O₂, 5% CO₂, and 80% N₂; 32° C.)Permeate Mem# pressure Permeance, mol/m²/s/Pa 60037- Feed gas (mbar) H₂OO₂ N₂ CO₂ CO₂/N₂ -56-55-2 CO₂ mix 8 2.6E−07 3.1E−07 5.0E−07 1.62-57-55-2 CO₂ mix 9 2.4E−07 3.1E−07 1.4E−06 4.61 -56-55-7 CO₂ mix 155.3E−06 4.4E−06 7.9E−06 1.81 -56-55-2 wet CO₂ 6 9.4E−06 1.5E−08 2.4E−081.3E−08 0.55 mix

VI. Exemplary Embodiment 4-Modification of NaA and Faujasite-TypeZeolite Membrane by Ion Exchange and Surface Reaction

Disclosed in this embodiment are methods that enhance permeance,selectivity, and/or stability. In one embodiment, a modification methodcomprises placing certain metal ions into the zeolite membrane frameworkby ion exchange to tailor the channel opening and chemistry of activesites. This approach utilizes a unique property of zeolite-typematerials, ion exchangeability. Different positively-charged ions can beintroduced into the framework to balance the negatively charged Al ionsin the tetravalent Si and Al—O bonded frameworks. The zeolite membraneas prepared is typically in Na⁺¹ form. The Na⁺¹ can be replaced by othermetal ions, such as, Ag⁺, Cu²⁺, Al³⁺, H⁺, Zn²⁺, Sr²⁺, Ca²⁺, K+, NH⁴⁺,Li⁺, La⁺³, Ce⁺³, Mg²⁺, Cs⁺¹, etc. Ion-exchange has been used to make thezeolite catalyst and adsorbent in powder form. Conducting ion exchangeof a zeolite membrane is very different from ion-exchange of the powderor particle material. The zeolite crystal in powder or particle form canbe readily stirred in a solution to do ion-exchange in a homogenousmixture, and the stress induced by ion-exchange is often not a problemfor particle or powder material. The zeolite membrane has a well-definedstructural surface and the stress induced by ion-exchange can causecracks of the membranes. In addition, the zeolite membrane has athin-layer of zeolite crystals and the depth of ion-exchange into themembrane layer can be controlled to generate a graded structure. Anexemplary ion-exchanging method to modify the thin-sheet zeolitemembrane is described as follows:

The zeolite membrane surface is contacted by a water-based solutioncontaining certain metal ions, while the backside of the membrane sheetis protected from exposure to the solution. The concentration of themetal ion in the solution is preferred to be 0.001 to 1.0 M, morepreferably, 0.001 to 0.1M. The solution concentration is chosen in sucha way that the ion exchange can be readily done without leaving muchresidual material on the membrane surface after ion exchange. Themembrane/solution contact is held above the freezing point and below theboiling point of the solution, preferably 10 to 60° C. The contactingtime is from 1 min to 1 day. The suitable contacting temperature andtime are chosen to avoid formation of cracks in the membrane due to thestresses generated during ion-exchange. After the exchange, theexcessive solution is drained from the membrane surface and the membranesurface is rinsed with de-ionized water. Then, the membrane is activatedby drying and/or heating at elevated temperatures.

The second modification method is about modification of exterior surfaceof the zeolite membrane by reacting surface hydroxyl groups with someother molecules containing silane and/or amine-functional groups. Thesurface modification changes chemistry of the exterior membrane surfaceand may not affect the zeolite channel opening. The surface modificationcan be conducted by either vapor-phase or liquid-phase deposition. Themembrane surface is exposed to the precursor molecules under suitableconditions such that the selective surface reaction occurs withoutblocking the zeolite pores. The liquid-phase modification is conductedat temperatures from 20 to 100° C. within 1 minute to a day. Thevapor-modification is conducted at 40 to 200° C. for 1 minute to 1 dayunder atmospheric pressures or vacuum.

EXAMPLES

i. Modification of NaA Membrane by Ion Exchange

A portion of 3.5 cm×3.5 cm uniform area was cut from the 13 cm×13 cmparent NaA membrane sheet prepared as previously-described. The membranecoupon was mounted on a magnetic tape to prevent the backside of themembrane sheet from contacting with the solution. The membrane couponwas placed inside a glass jar with the surface being faced up. 30 cc ofthe solution was poured into the jar to get the membrane surface fullycovered. The jar was covered and kept at the designated temperature.After a certain time, the membrane coupon was taken out and rinsed withtap water and de-ionized water. The membrane was left for drying insidethe fume hood and weight changes were measured. Table 26 lists theweight changes of the membrane coupons after different ion-exchangeconditions.

It is noted that the 0.1M Ag nitrate solution was covered with Al foilduring ion exchange to prevent its exposure to light. The membranesafter the ion exchange in the AgNO₃ solution (−81-3 and −81-8) were inpoor conditions. The supports were week and cracked in multiple areaswhen they were pulled off the magnetic tape. Further experimentationwith the Ag nitrate solution indicated that Ag ion was readily reducedby Ni into metallic Ag that was visible as white dots. Such a reductionreaction induced significant stresses in the Ni support structure, whichresulted in cracks. It was found that the membrane sheet maintained itsmechanical integrity when the Ag ion-exchanging time was shortened andthe membrane surface was thoroughly rinsed prior to the ion exchange.

Molecular separation functions of the ion-exchanged NaA membrane werecharacterized by air dehumidification tests. An air stream containing3.3 mol % H₂O was fed into the feed side of a membrane test cell, whilethe permeate side was pulled to vacuum at pressures in a range of 5 to35 mbar. The membrane having low H₂O/N₂ selectivity typically resultedin a high permeate side pressure. The membrane test cell temperature wasmaintained at 31-32° C. H₂O permeance and H₂O/N₂ separation factorcalculated from the measurement are listed in Table 26. It can be seenthat membrane performances are dramatically affected by ion exchangingconditions. The Cs⁺¹ ion-exchange substantially lowered H₂O permeance ofthe membrane without obtaining high H₂O/N₂ selectivity. The H⁺¹ion-exchange reduced the H₂O/N₂ selectivity. The NH₄ ⁺¹ ion-exchangeunder the present conditions caused significant membrane cracks. Themembrane ion exchanged with 0.1 M K⁺¹+Na⁺¹ solution at 22° C. maintainedgood permeance and selectivity. However, both permeance and selectivitydecreased when the ion exchange was conducted at 60° C. with the samesolution and exchanging time. The Ag⁺¹ ion-exchanged membrane showedconsistently high H₂O permeance and H₂O/N₂ selectivity. Ice bath wasused to collect the permeated water vapor.

TABLE 26 Ion exchange of thin-sheet NaA membrane with different metalion solutions Ion exchange conditions Membrane performances WeightH₂O/N₂ Mem ID Temp, Exchange change H₂O permeance separation 61509-Solution ° C. time, h wt. % mol/m²/s/Pa factor 81-1 0.1M CaCl₂ + NaCl 2218 0.06% 5.40E−06 50 (Ca:Na = 0.8:0.4) 81-2 0.1M KCl + NaCl 22 0.03%7.10E−06 518 (K:Na = 0.6:0.4) 81-3 0.1M Ag nitrate 22 3.49% Membrane wastoo weak to be tested 81-4 0.01M HCl 22 −0.94% 4.60E−06 3.2 81-5 0.1M Csnitrate 22 20 1.36% 7.00E−07 86 81-11 0.1M (NH₄)₂CO₃ 22 −4.15% Membranecracked 81-6 0.1M CaCl₂ + NaCl 60 18 0.20% 5.80E−07 0.6^(a) (Ca:Na =0.8:0.4) 81-7 0.1M KCl + NaCl 60 0.11% 2.00E−06 44.6^(a) (K:Na =0.6:0.4) 81-8 0.1M Ag nitrate 60 6.68% Membrane was too weak to betested 81-9 0.01M HCl 60 −1.38% 2.20E−05 3.5 81-10 0.1M Cs nitrate 60 201.25% 2.70E−07 35 81-12 0.1M (NH₄)₂CO₃ 40 −2.23% Membrane cracked 95-10.1M Ag nitrate 22 2 1.20E−05 831 95-2 0.1M Ag nitrate 22 4 1.10E−05 62495-3 0.1M Ag nitrate 22 6 1.20E−05 850 95-4 0.1M Ag nitrate 22 168.80E−06 628 85-5 0.1M Ag nitrate 60 16 9.20E−06 368

Surface textures and compositions of the ion-exchanged membranes wereanalyzed by SEM/EDS. Representative SEM pictures of different membranesare compared in FIGS. 45A-45L and the associated surface compositionsare listed in Table 27. The parent NaA/Ni membrane is included as acomparative basis. The Na ion in the membrane was gone after Ag ionexchange in only 2 hours. Further increasing exchange time did not makeany difference to the texture and surface composition. Most Na wasremoved after the membrane soaked in the HCl solution, and the zeoliteframework was destructed. Only fraction of K was exchanged into thezeolite. Increasing temperature did not enhance further exchange butcaused loss of Na and degradation of zeolite crystal structures. Thesimilar impact of temperature was observed for the Cs ion exchange. Somesmall cracks occurred with the Ca ion exchange. Membranes of low H₂O/N₂selectivity are typically associated with some cracks on the membrane.The Cs ion-exchanged zeolite at room temperature did not show any crack.Its low H2O permeance can be attributed to low H₂O transport rate in theCs ion-exchanged zeolite channel, which can be explained by reduced H₂Oadsorption and channel opening due to hydrophobicity and large size ofCs⁺¹ ion compared to Na⁺¹ ion.

TABLE 27 Surface compositions of ion-exchanged NaA/Ni membranes SampleID Atomic % 61509- Name Note C O Na Al Si M Ni 76-71-2 NaA/Ni Parentmembrane 58.36 16.78 13.08 11.78 81-1 Ca&Na @RT Some selectivity 62.0710.29 13.82 12.71 1.11(Ca) 81-6 Ca&Na @60° C. No selectivity 68.72 3.2413.31 12.47 2.27(Ca) 81-2 K&Na @RT excellent 7.25 58.17 7.81 12.63 11.412.73(K) selectivity 81-7 K&Na @60° C. some selectivity 4.72 57.84 9.3313.27 12.4 2.44(K) 81-4 NH₄ ⁺¹ @RT No selectivity 6.13 66.03 2.54 13.1710.81 1.32 95-1 Ag at 2 h Excellent 63.86 14.23 12.73 9.18 selectivity(Ag) 95-2 Ag at 4 h 64.14 14.18 12.87 8.82 (Ag) 95-3 Ag at 6 h 66.0513.23 11.85 8.09(Ag) 95-4 Ag at 16 h 63.96 14.35 12.97 8.73(Ag) 81-5 Cs@RT 63.87 5.17 14.44 13.78 2.73 (Cs) 81-10 Cs @60° C. 66.56 2.19 14.514.74 2.02 (Cs)

ii. Modification of NaA Membrane by Surface Reaction

In this example, surface modification methods were tested to modify aNaA membrane. The NaA/Ni membrane sheet as prepared often did not lookuniform over the whole sheet surface, and there were various features.The surface modification was attempted to fix some meso-defects on theas-prepared NaA membrane and enhance H2O/N2 selectivity. Table 28 listsfive membrane test coupons sampled from different areas of one membranesheet (#60037-13-6). The air dehumidification performances of the sametest coupon are compared before and after modification. The airdehumidification tests were conducted with atmospheric air containing3.4 mol % H2O in the feed side and 5-10 mbar vacuum in the permeateside. The membrane test cell temperature was maintained at 31-32° C. Thesurface modification was conducted by dropping the solution on themembrane surface, draining excessive solution off the membrane surface,and drying the membrane at room temperature. Two solutions used forimpregnation were 0.1M NaOH and 0.25 wt. % Nafion. Testing results ofthe coupons sampled from different areas of as-prepared NaA/Ni membranesheet did reveal significant variations in the membrane performance,which could be caused by defects and/or structure variations over thewhole area of a membrane sheet. In general, the membrane performance wassubstantially improved after the modification.

TABLE 28 Impacts of surface modification on air dehumidificationperformances of NaA/Ni membrane (testing coupon sampled from membranesheet 60037-13-6) Separation Permeance, mol/m²/s/Pa factor Test couponModification H₂O O₂ N₂ H₂O/N₂ #2. sampled from edge As-prepared 4.6E−068.8E−07 1.1E−06 3 #2. Modified impregnated with 0.1M 1.5E−05 4.2E−097.9E−09 899 NaOH #3. white streak in the center As-prepared 1.5E−051.2E−08 1.3E−08 560 #3. modified impregnated with 0.25 wt. % 1.1E−054.8E−09 5.4E−09 1153 Nafion solution #4. - three small black dotsAs-prepared 1.4E−05 7.4E−09 9.4E−09 752 #4. modified impregnated with0.25 wt. % 8.4E−06 6.2E−09 6.5E−09 774 Nafion solution #5. slightlydarker bands As-prepared 3.7E−06 9.2E−09 9.9E−09 217 #5. - modifiedimpregnated with 0.25 wt. % 1.2E−05 1.0E−09 1.5E−09 4244 Nafion solution#6. sampled from edge As-prepared 1.1E−05 1.0E−08 9.9E−09 613 #6.modified impregnated with 0.25 wt. % 2.6E−05 2.5E−09 2.8E−09 3362 Nafionsolution

Given the significant effects of surface modification on membraneperformances, test coupons sampled from another NaA/Ni membrane sheetwere used to compare different surface modification methods. In Table29, first four test coupons were sampled from the uniform area of thesame membrane sheet to compare impacts of the surface modification.Coupon #1 was tested as prepared. Coupon #2 was modified by soaking themembrane sample in Dow Corning® OS-2 Silicone solvent overnight anddrying at room temperature. This solvent is a volatile methylsiloxane(VMS) fluid developed specifically for use as a cleaning agent andcarrier. After modification, the membrane surface turned fromhydrophilic into hydrophobic, although the membrane surface looked same.Coupons #3 and 4 were modified using the impregnation methods asdescribed above. All the four membrane coupons showed excellentpermeance and H₂O/N₂ selectivity. The results indicate that thesesurface modification methods do not have negative impacts on the uniform(high quality) membrane surface, while they could be beneficial tofixing of some meso-defects. Fixing the meso-defects by the surfacemodification is confirmed with coupon #5. Both H2O permeance and H2O/N2selectivity were dramatically enhanced after modifying an as-preparedcoupon of relative low selectivity with the Nafion solutionimpregnation.

However, surface modification needs to be properly chosen to enhance themembrane performance. Test coupon #6 showed very low H2O/N2 selectivityafter it was modified by reacting with Tetraethyl orthosilicate (TEOS,reagent grade, 98%, Sigma Aldrich) vapor at 60° C. for 16 hours. Thismodification was done by hanging the membrane coupon in a closed glassbottle that had a layer of TEOS liquid on the bottom. The membranesurface was contacted with TEOS vapor that was vaporized from the TEOSliquid. By contrast, coupon #7 showed good permeance and selectivityafter modified with TEOS in a different way. The membrane surface wasexposed to liquid TEOS at 22° C. for 16 hours in a closed container.Then, the TEOS liquid was drained off the membrane surface and themembrane was left to dry in the room temperature. It is noted that bothcoupon #6 and #7 were hydrophobic after the modification, indicatingoccurrence of some surface reactions.

TABLE 29 Impacts of surface modification on air dehumidificationperformances of NaA/Ni membrane (testing coupon sampled from NaA/Nimembrane sheet #60037-11-5) Separation Permeance, mol/m²/s/Pa factorTest coupon Modification H₂O O₂ N₂ H₂O/N₂ #1. Uniform As-prepared1.2E−05 0.0E+00 0.0E+00 n/a #2. Uniform Soaked in OS2 solvent 1.2E−050.0E+00 0.0E+00 n/a #3. Uniform Impregnated with the 1.1E−05 0.0E+001.0E−09 13358 Nafion solution #4. Uniform Impregnated with the 1.1E−050.0E+00 1.6E−09 4978 NaOH solution #5. discolored As-prepared 9.1E−066.0E−08 7.6E−08 74 blemish #5. Modified Impregnated with the 2.1E−051.5E−08 2.1E−08 407 Nafion solution #6. Modified vapor deposition of1.1E−05 3.4E−07 4.1E−07 16 TEOS #7. Modified Soaked in TEOS 8.6E−060.0E+00 4.8E−09 1125

Surface textures of the membrane coupons listed in Table 29 wereanalyzed by SEM/EDS after air dehumidification tests, and shown in FIGS.46A-46G. The corresponding surface compositions are listed in Table 30.The membrane surfaces of modified coupons #2, 3, 4, and 5 look similarto the as-prepared membrane coupon. This confirms that the surfacemodification of present disclosure is meant to modify the surfacechemistry of the membrane exterior surface without altering its crystalstructures. Coupon #2 after OS2-modification shows significant presenceof C and no obvious Si enrichment, which indicates occurrence of surfacereaction between the membrane surface and OS2 solvent. The presence of Fand S atoms on coupon #3 and #5 confirms attachment of Nafion moleculesonto the membrane surface. There is no significant increase of Nacontent in coupon #4 after NaOH modification, indicating slight surfacemodification of the membrane by the NaOH solution. FIG. 46F showssignificant mud cracks on coupon #6, which were caused by excessivereaction of the membrane with TEOS molecules during CVD modification.The low H2O/N2 selectivity of this membrane coupon can be attributed tothese cracks. Significant presence of C atoms in coupon #7 is revealedafter the membrane surface was modified by moderate reaction with TEOSin liquid phase, which suggests attachment of TEOS molecule on themembrane surface. Since the membrane crystal structures are intact, thismembrane coupon maintained good H2O permeance and H2O/N2 selectivity.

TABLE 30 Surface compositions of modified NaA membrane (sheet ID60037-11-5) EDS At % Coupon # Probed area F S C O Na Al Si Ni #1. As-Spot 1 - representative 54.9 11.4 15.1 17.6 1.0 prepared area Spot 2 -loose particle 51.1 11.7 16.3 18.1 2.8 Spot 3 - densified area 60.9 10.013.0 15.5 0.6 #2. OS2 Full image 13.7 48.8 10.1 12.3 1e3.1 2.1 Modified#3. Nafion Full image 3.1 0.1 18.0 46.1 8.8 10.9 12.4 0.6 modified Spot1 - typical 2.9 0.0 17.8 46.9 8.9 10.8 12.0 0.7 membrane Spot 2 - largecrystals 0.7 0.0 10.4 54.5 9.7 11.4 12.7 0.5 Spot 3 - agglomerates 2.40.0 23.0 46.9 7.9 9.1 10.1 0.6 #4. NaOH Full image 17.4 48.4 8.6 11.213.5 0.8 modified #5. Nafion- Full image 0.6 0.1 10.9 50.7 9.9 12.6 14.20.9 modified #6. Modified Full image 48.3 12.9 14.6 15.4 8.8 by CVD ofSpot 1 - membrane 46.3 13.5 14.9 15.0 10.2 TEOS Spot 2 - agglomerates47.0 12.2 17.2 19.0 4.6 #7. Modified full image 39.3 39.9 5.5 7.1 7.70.5 with TEOS Spot 1 - membrane 13.2 49.7 9.6 12.4 14.3 0.9 impregnationSpot 2 - agglomerate 65.2 26.1 2.6 2.8 3.1 0.2

iii. Modification of NaA Membrane by Ion Exchange of Transition MetalIons

Modification of NaA membranes by ion exchange was further tested withtransition metal ions. 0.1M AgNO3, 0.1M CoCl2 and 0.1M Cu (NO3)3solutions were prepared. 3.5 cm×3.5 cm coupons were cut out of a NaA/Nimembrane sheet. The membrane coupon was mounted on a magnetic tape,loaded in a plastic jar, and immersed in a solution. The jar was placedin the Lab-Line Environ Shaker Mixer, and shaken at 30° C. for 2-4hours. Then, the membrane was taken out of the jar, rinsed once withde-ionized water, and dried inside the vacuum oven at 120° C. overnight.The preparation conditions are summarized in Table 31.

Molecular separation performances of the modified membranes werecharacterized by gas permeance measurements of a few gas streams, air,CO2, 16.7% CO2 in air mixture, and 3.3% H2O and 16.7% CO2 in air. Thefeed gas was introduced to the membrane side under atmospheric pressure,while the permeate side of the membrane was pulled vacuum to 5 to 10mbar. Table 32 lists the molecular permeance values measured. Permeancefor a molecule may vary dramatically among different gas compositionstested because different molecules can have very different adsorptionand transport properties in the zeolite membrane. For example, H₂Opermeance dominates with a humid feed gas.

TABLE 31 NaA membrane exchanged with transition metal ions Couponorienta- Ex- Weight Parent tion in change gain, Mem ID Mem ID Solutionexchange time, h wt. % 61794-13-Ag 60037-89-2 0.1M face up 4 1.84 AgNO361794-13-Co 60037-89-2 0.1M CoCl2 face up 4 −0.11 61794-13-Cu 60037-89-20.1M face up 4 −0.06 Cu(NO3). 61794-16-Ag 60037-89-2 0.1M face down 22.10 AgNO3 61794-16-Co 60037-89-2 0.1M CoCl2 face down 2 0.1661794-16-Cu 60037-89-4 0.1M face down 2 −0.15 Cu(NO3).

TABLE 32 Gas permeance (mol/m²/s/Pa) of modified NaA membranes Mem IDAir CO₂ CO₂/air mix Humid CO₂/air 61794- T, ° C. O2 N2 CO2 CO2 O2 N2 H2OCO2 O2 N2 16-Ag 30 4.0E−07 4.4E−7 2.1E−8 3.4E−07 3.9E−07 4.4E−07 6.6E−060.0E+00 1.8E−07 2.1E−07 Aged 30 0 3.4E−07 3.6E−07 6.4E−06 0 1.1E−087.3E−09 65 0 3.9E−8 6.6E−06 0.0E+00 1.8E−07 2.1E−07 16-Co 30 3.7E−063.5E−6 3.2E−6 3.9E−06 3.9E−06 3.8E−06 3.5E−06 2.0E−06 2.1E−06 2.2E−06 653.8E−06 3.7E−6 2.7E−6 3.7E−06 3.7E−06 3.8E−06 16-Cu 30 1.9E−05 1.5E−55.1E−6 1.8E−05 2.0E−05 1.7E−05

Molecular sieving functions of the Ag ion-exchanged membrane(61794-13-Ag) were tested over a range of conditions. FIG. 47A showsvariations of H2O, CO2, O2m and N2 permeance with temperature with aconstant feed gas (3.3% H2O, 15% CO2, balance air). The feed gas wasunder atmospheric pressure, while the permeate was in 5 to 10 mbar.Excellent selectivity of this membrane to H2O over other molecules isshown. At temperatures below 40° C., air permeation could not bedetected. CO2, N2, and O2 permeance increased as the temperature wasincreased to 65° C. However, permeance values of these molecules remainabout 2 to 3 orders of magnitude lower than the H2O permeance. After 65°C.-testing with the humid CO2/air mixture, the membrane was tested withdry 15% CO2 in air. O2 and N2 permeance could not be detected over atemperature range of 30 to 78° C. The CO2 permeance was fairly low inthe order of 1E-10 mol/m2/s/Pa. Then, the temperature was raised to 124°C. and tested with feed gases of different H2O molar fraction. FIG. 47Cshows that under the constant temperature of 124° C., H2O permeancestayed approximately same as H2O molar fraction in the feed gas wasvaried from 3.3 to 20 mol %. The H2O permeance at such a hightemperature is about 3 orders of magnitude higher than the N2 and O2permeance. This Ag-exchanged NaA/Ni membrane is demonstrated to be anexcellent H2O-permeable membrane over a range of conditions.

As a comparison, the parent NaA membrane, which was used to make the Agion-exchanged membrane, was tested over a range of conditions. FIG. 48Ashows that H2O and CO2 permeance are not much affected by increasing CO2molar fraction in the feed gas under a constant temperature of 63° C. Atabout the same temperature with the same feed gas composition (15% CO2,3.3% H2O, balance air), the Ag ion-exchanged membrane shows about oneorder of magnitude higher H2O/N2 selectivity than the parent NaAmembrane. Comparison of FIG. 48B to FIG. 47B shows that dry gaspermeance through the NaA membrane is about 3 orders of magnitude higherthan the Ag ion-exchanged one. The possibility to dramatically enhancethe membrane H2O/air selectivity by ion exchange is demonstrated.

In FIGS. 49A-49C, SEM pictures of the parent NaA membrane surfaces arecompared to the ion-exchanged one. Corresponding surface compositionsanalyzed by EDS are listed in Table 39. The micro-structures ofas-prepared NaA membranes look same before and after the gas separationtests with different gases at 63° C. The atomic compositions are alsosimilar. This is as expected, since the membrane structure should not beaffected by such moderate testing conditions. The Ag ion-exchangedmembrane showed same micro-structures as the parent NaA membrane evenafter tests with various gases at 124° C. Presence of some bright spotson the membrane surface is believed to be Ag particles. EDS analysis ofdifferent spots on the Ag ion-exchanged membrane reveals that Na ion inthe original zeolite framework was completely exchanged by Ag ion. TheSEM/EDS results confirm that the Ag ion-exchange modified the zeolitechannel without changing the crystal structure of the membrane. It islikely that because Ag ions are larger than Na ions, its inclusion intothe zeolite framework reduces the zeolite pore opening and blockspermeation of N2, O2, and CO2. H2O molecules may be transported throughthe zeolite channel by association with Ag ions.

TABLE 39 Surface compositions of the NaA membranes as-prepared, tested,and ion-exchanged EDS At % Membrane area Spectrum # O Na Al Si Ag Al/SiNa/Al 89-2 NaA as-prepared impure materials 1 63.24 10.91 10.83 15.020.72 1.01 large crystal 2 61.09 16.74 9.72 12.46 0.78 1.72 Uniform area3 54.42 15.98 12.45 17.15 0.73 1.28 89-2 NaA tested Uniform area 1 57.116.2 12.6 14.1 0.89 1.29 impure materials 2 54.0 21.7 10.9 13.4 0.821.98 Ag ion-exchanged one tested uniform 1 45.33 16.67 21.61 16.39 0.770.98 deposited crystal 2 46.70 16.18 21.07 16.06 0.77 0.99 impurities 359.63 12.44 17.76 10.16 0.70 0.82 embedded crystal 4 42.27 17.20 21.8718.67 0.79 1.09 embedded crystal 5 51.69 15.29 19.73 13.28 0.77 0.87

Because of the exceptional performances of the Ag ion-exchangedmembrane, more ion exchange experiments were performed with dilute AgNO3solution. The ion exchange was conducted at 23° C. for only 15 minute.After ion exchange, the membrane was dried in a vacuum oven at 120° C.for 15 hours. 0.3% and 1.6% weight gains were obtained with 0.006 and0.025M Ag nitrate solution, respectively. The studies show that the Agion exchange of the NaA membrane is rapid. FIGS. 50A and 50B show themolecular separation performances of the 0.025M Ag ion-exchangedmembrane (ID61794-41-4). With a dry feed gas comprising 17% CO2 in air,N2 and CO2 permeance was essentially too low to be detected as theseparation temperature was increased from 60 to 120° C. There was someO2 permeance. It decreased with increasing temperature until 100° C. andthen increased as the temperature was raised to 115° C. The resultsconfirm low permeance of the Ag ion-exchanged membrane to dry gases. Ata constant temperature of 115° C. with humid feed gas, H2O permeance is2-3 orders of magnitude higher than O2 permeance, and CO2 and N2permeance remained beyond the detection limit. The selective H2Opermeance of Ag-exchanged NaA membranes is confirmed.

For comparative purposes, a commercially-available Nafion membrane sheetwas acquired and tested. The Nafion membrane is known for its H2Opermeance. FIGS. 50A and 50B and FIGS. 51A and 51B show the gaspermeance at different temperatures and with feed gas of different H2Ocontent. With a feed gas containing 3.3% H2O, both H2O and N2 permeancedecreased with increasing temperature from 30 to 115° C. At 115° C.constant temperature, H2O permeance decreased with increasing H2Ocontent in the feed gas while N2 and O2 permeance stayed at a lowlevel—close to the detection limit. Compared to the Ag ion-exchangedzeolite membrane (see FIGS. 50A and 50B), the Nafion membrane (see FIGS.51A and 51B) shows about 2 orders of magnitude H₂O permeance than theAg-exchanged one at elevated temperatures (<80° C.). At low temperature(31° C.), the Nafion membrane shows about 3 times lower H₂O permeancethan the Ag ion-exchanged one. At this temperature, the Ag ion-exchangedmembrane showed nearly infinite selectivity to H₂O over air (O₂ or N₂),while the Nafion membrane showed H₂O/O₂ selectivity about 200.

iv. Zeolite Membrane Surface Modification with Molecules ContainingAmine Function Groups

The thin-sheet zeolite membranes were modified through reaction ofsurface hydroxyl groups with molecules containing amine functionalgroups. 3.5 cm×3.5 cm test coupons were cut from representative NaA/Niand NaX/Ni membrane sheets. The three molecules tested were3-(2-Aminoethylamino)propyl]trimethoxysilane (2-APTMS, Sigma-Aldrich),(3-Aminopropyl)triethoxysilane (assay=98%) (APTES, Sigma-Aldrich), and3-aminopropyldimethylethoxysilane (APDMES, Gelest Inc.). The proceduresfor modification with a water-based solution are as follows. First, 10wt % of APTES and APTMS solutions were prepared in the de-ionized water.The membrane coupons were placed in a closed container and soaked in thesolution. The container was placed inside an oven to heat at 75° C. for2 hours. Then, the membrane was rinsed with de-ionized water and in thevacuum oven at 120° C. overnight. The modification with an organicsolvent-based solution is as follows. 4 wt. % of APDMES solution wasprepared in toluene. A membrane coupon was loaded into a Teflon jar andimmersed in the solution. The coupon was soaked at room temperature inN2 environment for 1 day. Then, the membrane was rinsed by methanol anddried in the vacuum oven at 110° C. for 1 hour. The results aresummarized in Table 40. As expected, the weight change of the membranecoupon by the surface reaction was minimal. The modified membrane waschecked by the colored water test. All the membranes listed in Table 40showed no color leakage.

Gas permeance through the modified membrane was measured with differentfeed gases: air, pure CO2, 16.7% CO2 in air, 3.3% H2O and 16.7% CO2 inair. The results are summarized in Table 41. The NaA membrane modifiedthe APDMES/toluene solution (16-A-DMES) showed no permeance to CO2, N2,and O2 for a humid feed gas, although the permeance of dry gases wassignificant. The Faujasite membranes modified with respectiveAPDMES/toluene (16-X-DMES), APTMS/H2O (16-X-TMS) and APTES/H2O(16-X-TES) solutions showed no or little permeance to dry gases. Inparticular, membrane #16-X-DMES showed very good H₂O permeance and nopermeance of CO₂, N₂ or O₂ with the humid feed gas. As taught in thepreparation of Faujasite zeolite membrane, Faujasite-type framework hasmuch larger pores than NaA-type framework. The present example showsthat selectivity of the Faujasite membrane to H2O over other moleculescan be dramatically improved by proper surface modification.

TABLE 40 Thin-sheet NaA-type and Faujasite-type (NaX) membranes modifiedby surface reaction Parent Modification conditions Weight Mem IDmembrane Solution, Temp, Drying, change Color 61794 Type ID 60037-Solution ml ° C. Time, h ° C./h wt % leak test -16-X TES NaX 81-5 APTESin 10 75 2 120/12 0.06 no leak H2O -16-A TES NaA 89-4 APTES in 10 75 2120/12 −0.10 no leak H2O -16-X NaX 83-77-3 APTMS 10 75 2 120/12 −0.34 noleak TMS in H2O -16-A NaA 89-4 APTMS 10 75 2 120/12 −0.68 no leak TMS inH2O -16-X NaX 81-5 APDMES 10 22 24 110/1  0.03 no leak DMES in Toluene-16-A NaA 89-4 APDMES 10 22 24 110/1  0.00 no leak DMES in Toluene

TABLE 41 Gas permeance of the zeolite membrane modified with moleculescontaining amine function groups (separation temperature of 30° C., gaspermeance in unit of mol/m²/s/Pa) Mem ID Air CO₂ CO₂/air mix HumidCO₂/air 61794- O₂ N₂ CO₂ CO₂ O₂ N₂ H2O CO2 O2 N2 16-A- 2.3E−07 2.6E−74.4E−7 4.4E−07 4.6E−07 5.1E−07 2.5E−06 0 1.3E−08 7.7E−09 DMES 16-X-8.4E−09 1.7E−9 1.4E−9 1.3E−08 0 0 5.9E−06 0 0 0 DMES 16-X-TES 1.7E−09 0ND ND ND ND 4.3E−07 0 00  00  16-X-TMS 1.2E−08 3.1E−9 1.9E−10 ND ND ND2.5E−06 0 0 0

VII. Exemplary Embodiment 5—Thin-Sheet MFI-Type Membrane Structure andPreparation A. Introduction

MFI-type zeolite has a pore size about 0.57 nm, which falls between theNaA-type and Faujasite-type zeolite pores, and is an important class ofzeolite materials with a variety of industrial applications. Compared tothe other zeolites, MFI-type zeolite has one unique attribute that itsphysical and chemical properties can be tuned by changing Si/Al ratio inthe framework over a wide range from about 10 to infinity. A MFI-typezeolite without any Al (Si/Al ratio=infinity) has a framework assilicalite. Silicalite is a highly hydrophobic material that does notadsorb water molecules. Affinity to water molecules is increased withintroduction of Al into the framework. Thus, selectivity of the MFI-typezeolite membrane to H₂O over other molecules can be tuned by varyingSi/Al ratio in the framework. For example, the membrane without Al willbe selective toward ethanol over water molecule, while the membrane withAl will be selective toward water over ethanol.

MFI-type zeolite membranes have been extensively studied in theliterature for molecular and gas separation, such as CO₂ separation fromCO₂-gas mixtures, H₂ separation from H₂ gas mixtures, separation oflinear molecules from the branched ones, and ethanol/water separation.Those membranes were typically prepared on ceramic disks or tubes viaeither direct growth or secondary growth. In addition to fragility andcost issues associated with those types of membrane supports, Al atomsin the ceramic support can be readily incorporated into the zeoliteframework during hydrothermal growth so that it becomes impossible toobtain the zeolite membrane free of Al atoms.

B. Description of Exemplary MFI-type Membranes

The MFI-type membrane structures of present invention are the same tothe thin-sheet NaA membrane described previously. Exemplary preparationprocess steps are outlined in FIG. 52. Selection of suitable thin-sheetsupport, seed coating, and secondary growth are the same process stepsas used for preparation of NaA and Faujasite-type membranes. MFI-typecrystals with particle sizes from 0.1 to 2 μm are used as the seedingcrystal. A solution suitable for MFI-type zeolite crystal growth is usedfor secondary membrane growth. Different from NaA and Faujasite-typemembranes, one additional process step needed for MFI-type membranepreparation is removal of organic template. An organic template is oftenadded into the growth solution to obtain high quality MFI-typemembranes. The template needs to be removed to open up the channel poresfor separation. The template often needs to be removed at elevatedtemperatures. Its removal must be controlled carefully to avoid cracksof the zeolite membrane. Previously, it has been disclosed to remove thetemplate by calcination at 500° C. in O₂-containing environment. In thepresent example, a moderate process is taught for removal of the organictemplate. One way is to expose the membrane surface only to an oxidizingenvironment by protecting the support, such as 1 to 5 mol % O₂ in aninert gas, at a temperature below 500° C., preferable 350 to 450° C.,for a time period of 1 to 24 hours. In this way, oxidation of thetemplate in the zeolite membrane is allowed to take place gradually andto eliminate formation of hot spots from exothermic oxidation reactions.Another way is to expose the membrane sheet to a reducing gasenvironment, such as H₂-containing gas, at temperatures up to 600° C.,to allow thermal decomposition or hydrolysis of the organic template fora time period of 1 to 20 hours. The heating and cooling is controlled at1° C./min or less.

The MFI-type membrane containing Al is selective toward H₂O over othermolecules and can be used for removal of water molecules fromwater-containing mixtures as NaA-type and Faujasite-type membranes. Onenotable feature with the MFI-type membrane without Al, i.e., silicalitemembrane, is selective permeation of hydrocarbon molecules over watermolecules. The silicalite membrane can be used to remove alcohols orhydrocarbon molecules from water-containing mixtures. One of suchapplication examples is concentration of ethanol from fermentationbroth. The fermentation typically produces a dilute ethanol streamcontaining 1 to 15 wt. % ethanol. Concentrating such a dilute ethanolstream to above 50 wt. % by distillation is a very energy-intensiveprocess. An ethanol-selective membrane can be used for selective removalof ethanol from dilute ethanol/water mixtures as illustrated by theprocess flow diagrams shown in FIGS. 53A and 53B.

As a clarified ethanol/water stream flows over the membrane surface,ethanol is selectively removed into the other side of the membrane. Thetransport of ethanol across the membrane is driven by difference inethanol partial pressure or chemical potential between the two sides ofthe membrane. The feed stream is typically at or above atmosphericpressure. The difference can be generated by pulling vacuum on thepermeate side of the membrane or by sweeping the permeate with a fluidstream.

C. Examples

i. Preparation of Thin-Sheet MFI-Type Membrane

The raw materials used to prepare MFI-type membrane in this example were1.0 M TPAOH (Sigma-Aldrich), tetraethyl orthosilicate (TEOS, 99%,Sigma-Aldrich), sodium hydroxide (99%, Merck), and sodium aluminate(anhydrous, Sigma-Aldrich).

The seeding crystals were prepared by conducting solution growth undermoderate conditions. A growth solution with composition ofTEOS:TPAOH:H₂O:NaOH molar ratio=1.0:0.152:24.7:0.008 was prepared. 0.094g NaOH was dissolved in 86.49 g H₂O to form a NaOH solution, to which 45cc of TPAOH solution was added. The mixture was stirred at roomtemperature until the solution became clear. Under stirring, 61.51 g ofTEOS was gradually added. The mixture was stirred at room temperatureovernight to obtain a clear solution. The clear solution was heatedunder stirring at 85° C. for 3 days. The solution turned milky. Thesolid was separated out of the solution by centrifuge at 4500 RPM. Thesolid precipitate was rinsed with de-ionized water three times.Resulting wet cake was divided into two portions. First portion wasdried in the oven at 120° C. overnight and calcined in a furnace at 500°C. for 5 hours at 1° C./min ramp, which is denoted as calcined seeds andassumed to be free of organic template. Another portion was dried atroom temperature and denoted as as-synthesized seeds.

FIGS. 54A and 54B show morphologies of the seeding crystals prepared inthis work. The calcined sample looks the same as the as-synthesized one,because the calcination only removed the template in the zeolitechannel. The seeding crystals comprise distinctive zeolite crystals ofabout 100 to 300 nm sizes.

TABLE 42 Elemental compositions of seeded porous Ni supports in SEMimages of FIGS. 55A and 55b Sheet coated with Sheet coated 0.15 mg/cm²of with 0.24 mg/cm² of calcined seeds (FIG. 55A) calcined seeds (FIG.55B) Spot 4 3 2 1 4 3 2 1 O at % 10.41 54.84 62.00 57.84 36.08 63.5763.32 59.46 Si at % 3.04 19.93 36.97 32.74 16.75 31.88 35.72 32.20 Ni at% 86.30 14.07 0.36 7.39 46.69 4.05 0.24 7.88

Ten grams of as-synthesized seeds was mixed with 70 g de-ionized waterand 30 cm³ zirconia beads in a 125 ml PP bottle, and ball-milled at 50%of the full-scale speed for 2 hours. The milled slurry was added withde-ionized water to obtain a seed coating solution with 5.93 wt. % solidloading of the as-synthesized seeds. 5.10 g of the calcined seeds wasmixed with 70 g de-ionized water and 30 cm³ zirconia beads in a 125 mlPP bottle, and ball-milled at 50% of the full-scale speed for 9 hours.The milled slurry was added with water to obtain a seed coating solutionwith 3.0 wt. % of the calcined seeds.

13 cm×13 cm porous Ni sheets were coated with the above seed coatingsolution on the spay coater in the same way as used for the NaA membranepreparation. For each support sheet, two times of coating was performedto obtain a visually-uniform seeded support surface. For each seedingmaterial, two different seed loading levels were made by controllingspray time to assess impacts of the seed loading on the membrane growth.

TABLE 43 Elemental compositions of seeded porous Ni supports in SEMimages of FIGS. 55C and 55D. 0.18 mg/cm2 of 0.27 mg/cm2 ofas-synthesized as-synthesized seeds (FIG. 55C) seeds (FIG. 55D) Spot 4 32 1 3 2 1 O at % 10.41 54.84 62.00 58.30 25.15 64.45 58.30 Si at % 3.0419.93 36.97 31.86 11.36 34.72 32.91 Ni at % 86.30 14.07 0.36 7.32 63.350.22 8.30The seeding crystal dispersion on the four seed-coated support sheets isshown in FIGS. 55A-55D. There is no continuous seed coating layer formedon these supports. Different spots of a seeded support surface wereanalyzed by EDS to determine the presence of seeding crystals. Table 42lists elemental compositions of the seeded sheets shown in FIGS. 55A and55B, while Table 43 lists elemental compositions of the seeded sheetsshown in FIGS. 55C and 55D, Presence of Si and O was found in the spotthat looks bare, because the seeding crystals can sit inside the supportpores. The higher the seed loading, the higher Si content is measured.It appears that the as-synthesized seeds provided more uniformdispersion of the seeding crystals than the calcined one. This may becaused by presence of more agglomerates in the coating solution made ofthe calcined seeds.

Two growth solutions with different compositions were evaluated. TheTEOS:TPAOH:H₂O:NaOH molar ratio was 1.0:0.120:68.1:0.008 for solution 1and 1.0:0.152:24.7:0.008 for solution 2. The growth solution wasprepared in the procedure as described above. The growth solution wasclear prior to growth.

The seeded support sheets were loaded into the planar reactor and fullyimmersed in the growth solution. The reactor was heated to thedesignated temperature at 1° C./min and held for 2 hours, and cooleddown naturally. The membrane sheet unloaded from the reactor was rinsedand dried. The growth weight gain was recorded.

Table 44 summarizes three batches of membrane growth runs with identicalseeded sheets. Runs 1 and 2 were conducted with solution 1 at differentholding temperatures. One the same seeded support, more weight wasgained with the higher growth temperature. Run 3 was conducted at thesame growth temperature as run 2 but with solution 2. Solution 2 wasmore concentrated than solution 1. It can be seen that under the samegrowth conditions, growth weight gain with the higher concentration isless than that with the lower concentration. Thus, significantpenetration of the growth solution into the support pores may occur withthe low concentration. In run 3, more growth gain was shown with thehigher seed loading level, which suggests that the growth dominantlyoccurred on the seeded surface. Thus, growth penetration into thesupport pores can be controlled by the growth solution concentration.

TABLE 44 MFI-type membranes prepared under different conditions Run 1:Run 2 Run 3: Seed coating Solution 1 at 120° C. h Solution 1 at 140° C.2 h Solution 2 at 140° C. 2 h loading, Mem ID Growth gain, Growth gain,Mem ID growth gain, Seeds mg/cm² 60037- mg/cm² Mem ID 60037- mg/cm²60037- mg/cm² calcined 0.15 99-97-3 2.0 97-3 2.6 112-105-1 1.31 0.2499-97-4 2.3 97-4 3.3 112-105-2 1.50 as- 0.18 99-95-1 2.3 97-95-1 2.5112-105-3 0.98 synthesized 0.27 99-95-2 2.0 97-95-2 2.3 112-105-4 1.56

All the membrane samples listed in Table 44 passed the color leakagetests, indicating no meso- or macro-defects. The gas permeance ofas-grown membranes was measured with CO₂ and air. As-grown membranesshowed no or little gas permeation at 31° C. because the membrane poreswere occupied by the template.

The membrane structures were analyzed by SEM/EDS and XRD. Table 45 listssurface compositions of a few membranes grown at 120 and 140° C. Ni, Siand O are the only elements detected by EDS, suggesting a membrane layerof pure SiO₂. It is noted that EDS is not an accurate method to measureO content. The 140° C.-grown membranes generally show a lower Ni at %than the 120° C.-grown one, which is consistent with the more zeolitegrowth at the higher temperature. Structures of the membranes areillustrated with three membrane samples grown at 120° C. in FIGS.56A-56F. A continuous-inter-grown zeolite membrane layer was observedwith all the membranes. The deposit on the membrane surface likelyresulted from adsorption of crystals and particles from the bulk growthsolution onto the membrane surface. The dense, continuous membrane layeris clearly revealed by the cross-sectional analysis. Some zeolitepenetration into the support pores is evident.

The crystal phase of the membranes was confirmed by XRD analysis. FIG.57 shows XRD patterns of the calcined seed and two membranes grown withit. XRD of the seeding crystals was measured in powder form, while themembrane was measured in the thin-film form. The major peaks at 2θ>43°are attributed the Ni support. The membrane XRD patterns match those ofthe seeding crystal well. The XRD patterns of the as-synthesized seedand the membrane grown out of it are compared in FIG. 58. The peaks ofthe membrane well match those of the seeding crystal. Crystal phaseidentification with the database shows that the MFI-type framework isthe dominating crystal phase for the seeds and membranes.

This example demonstrates that the MFI-type membrane can be grown on theseeding crystals either as synthesized or calcined even at growthtemperature of 120° C.

TABLE 45 Surface compositions of MFI-type membranes prepared in thisexample Mem ID Seeding Growth O at Si at. Ni at 60037- crystals temp, °C. Spot % % % 99-97-3 calcined 120 Full image 55.7 41.5 2.78 99-97-4calcined 120 Full image 55.8 42.0 2.25 99-97-4 calcined 120 Spot 1 58.540.7 0.80 Spot 2 63.3 35.7 1.00 99-95-1 as-synthesized 120 Full image56.5 42.7 0.84 99-95-2 as-synthesized 120 Full image 56.8 41.8 1.39 97-3calcined 140 Full image 56.3 42.8 0.88 97-95-1 as-synthesized 140 Fullimage 56.5 42.7 0.84

ii. Ethanol/Water Separation with MFI-Type Zeolite Membrane

The silicalite membrane (#6109-1-2) grown at 140° C. was calcined in 2%O₂/N₂ gas at 400° C. for 4 hours at 1° C./min ramp rate. The membranewas tested for ethanol/water separation. 10 wt % EtOH in water wasintroduced into the feed side of a membrane test cell at flow rate of 1cc/min under 2.2 bar. The membrane cell was maintained at 75° C. Thepermeate side of the membrane cell was swept by 500 sccm of Helium gasflow at 1.1 bar. Helium gas effluent from the membrane cell passedthrough a liquid nitrogen cold trap where the permeated water andethanol was condensed and collected. The flux and separation factor werecalculated from the experimental measurements by use of the followingequations:

$\begin{matrix}{J = \frac{\Delta\; W_{P}}{\Delta\;{t \cdot {SA}_{M}}}} \\{{SF} = \frac{\frac{y_{et}}{1 - y_{et}}}{\frac{x_{et}}{1 - x_{et}}}} \\{{J = {flux}},{{{kg}/m^{2}}/h}} \\{{{\Delta\; w} = {{weight}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{permeate}\mspace{11mu}{collected}\mspace{14mu}{at}\mspace{14mu}{time}\mspace{14mu}{interval}\mspace{14mu}{of}\mspace{14mu}\Delta\; t}},{kg}} \\{{{\Delta\; t} = {{sampling}\mspace{14mu}{time}}},h} \\{{{SA}_{m} = {{area}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{membrane}\mspace{14mu}{test}\mspace{14mu}{coupon}\mspace{14mu}{exposed}\mspace{14mu}{to}\mspace{14mu}{the}\mspace{14mu}{feed}}},\; m^{2}} \\{{SF} = {{{Ethanol}/{water}}\mspace{14mu}{separation}\mspace{14mu}{factor}}} \\{y_{ET} = {{ethanol}\mspace{14mu}{fraction}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{permeate}}} \\{x_{ET} = {{ethanol}\mspace{14mu}{fraction}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu}{feed}}}\end{matrix}$

FIG. 59 shows variations of flux and ethanol/H₂O separation factor withtime on stream. The flux increased with time and reached plateau about3.1 kg/m²/h, while the separation factor declined and reached a plateauaround 4.5. The pore in the as-prepared membrane may be blocked by someloose structures or matters. As these temporary features were flushedaway during the test, the permanent membrane structure was exposed sothat the flux and selectivity were stabilized.

Another silicalite membrane (#030110-1) grown at 140° C. and heated at400° C. was tested for ethanol/water separation by pervaporation. 10 wt.% ethanol in water was fed into the feed side of the membrane test cellat 3 cc/min under atmospheric pressure, while the permeate side waspulled vacuum to about 1-2 mbar. The permeate was collected in a liquidnitrogen cold trap. FIG. 60 shows that flux slightly declined with timeand then increased to reach a plateau, while the separation factordeclined with time and reached a plateau. The trends suggest that sometemporary structures obstructing membrane pores may have been removedwith the time. This membrane showed a very high flux, about 11 kg/m²/h,after 25-hours on the stream.

Impacts of silicalite membrane preparation on ethanol/water separationare shown by the following group of experiments. The silicate membraneswere grown with two solutions of different H₂O/Si molar ratios while theSi/TPAOH ratio being kept at 0.123. The growth conditions were 140° C. 2hours at 1° C./min ramping rate. The following methods were utilized toremove the template:

-   -   Method 1: 2% O₂/N₂ gas flow, 1° C./min to 400° C., 4-h at 400°        C., cooling down at 1° C./min.    -   Method 2: inert gas purge (He), 1° C./min to 400° C., 10-h at        400° C., cooling down at 1° C./min.    -   Method 3. 2% O₂/N₂ gas flow, 1° C./min to 360° C., 10 h at 360°        C., cooling down at 1° C./min.    -   Method 4. Conduct pervaporation testing of as prepared membrane        sample at 75° C. with pure ethanol first, monitor the permeation        flux

Method 1 was considered as a standardized procedure for removal of thetemplate from the thin-sheet MFI membrane identified in this work. Inmethod 2, only inert gas stream was used with a longer exposure time at400° C. In method 3, the holding temperature was lowered to 360° C.while the holding time was prolonged to 10 hours. Method 4 was a test ofpossible extraction of the template by ethanol at the pervaporationtemperature. The resulting membranes were tested for ethanol/waterseparation by pervaporation under the same conditions as describedabove.

The results are summarized in Table 46. Method 1 remains effective forremoval of the template. Fairly high flux and good ethanol/waterseparation factor were obtained. The membrane pore could not be openedup by flushing the membrane surface with pure ethanol under thepervaporation condition, as evidenced by very small flux in 7.5-hourflushing. It is interesting to note that methods 3 and 2 are alsoeffective for removal of the template, as evidenced by the high flux andgood ethanol/water separation factor obtained.

This example demonstrates that a silicalite membrane for selectiveethanol/water separation can be grown under moderate conditions (140°C.) with different concentrations of the growth solution, and thetemplate can be removed by exposing the membrane surface to a gasenvironment containing no or a little amount of oxygen at temperaturesabout 400° C. The thin-sheet membrane maintained its mechanicalintegrity after such treatment. This heating temperature is much lowerthan 500° C.-calcination that was typically reported in the art.

TABLE 46 Impacts of membrane preparation on ethanol/water separationperformances Membrane preparation Growth Separation performancessolution Template Time on Flux, Ethanol/water Concentration Mem# H2O/Siratio removal stream, h kg/m²/h separation factor factor 32910-1 79Method 1 1 5.4 4.3 3.1 4 5.6 5.6 3.7 32910-2* 8 7.1 5.3 3.5 Method 4 7.50.007 Pure ethanol 32910-3 40 Method 3 1 3.77 7.0 4.1 4 3.47 4.7 3.332910-4 8 4.11 4.8 3.3 Method 2 2 13.33 3.3 2.58

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention. Rather, thescope of the invention is defined by the following claims. We thereforeclaim as our invention all that comes within the scope and spirit ofthese claims.

I claim:
 1. A hydrocarbon-permeable membrane sheet for selective removalof hydrocarbon molecules from a mixture comprising: ahydrocarbon-selective membrane layer comprising inter-grown MFI-typezeolite crystals comprising SiO2/Al2O3 and disposed on a support surfaceof a porous metal-based support sheet, wherein the porous metal-basedsupport sheet is less than 200 μm thick and is substantially free ofsurface pores greater than 10 μm in diameter, wherein substantially freeis less than 1% of the surface, wherein the membrane layer has thicknessabove the support surface in the range of about 2 to 20 μm and has apenetration depth below the support surface in the range of about 1 μmto about 10 μm, wherein the permeable membrane sheet has a hydrocarbonpermeance greater than 1×10-6 mol/m2/Pa/s and a hydrocarbon selectivitygreater than 2, and wherein the membrane sheet is bendable from asubstantially flat configuration to a configuration having a radiusgreater than ½″ (1.3 cm) without cracking.
 2. The membrane sheet ofclaim 1, wherein the hydrocarbon-selective membrane layer is configuredto remove alcohols from alcohol-water mixtures.
 3. The membrane sheet ofclaim 1, wherein the porous metal-based support sheet has a porositybetween 15 and 55%.
 4. The membrane sheet of claim 1, wherein the porousmetal-based support sheet has a porosity between 30 and 45%.
 5. Themembrane sheet of claim 1, wherein the porous metal-based support sheetcomprises greater than 60% by weight of Ni, an Ni alloy, or stainlesssteel.
 6. The membrane sheet of claim 1, wherein the MFI-type zeolitecrystals have a SiO2/Al2O3 molar ratio greater than 100.